This is how 'I' learned that...

The "today I learned" thread is similar to this, and there might be some crossover, but I want to hear your stories of how you "learned" to do the things we all should know to do anyway. For example, tell the story of how you learned to 'measure twice cut once', or that lug nut torque is important, or that if you say you will call you need to call. I don't just want to know what you learned, I want to know HOW you learned it.

I'll go first. I learned not to stand behind a table saw (and probably, to not use a tablesaw like an idiot):

A few years ago, I had a buddy come to my house for a week and help me replace all the windows (he's a carpenter by trade). After we got all the windows in, we were trimming the outsides. He was up on the ladder, and yelling measurements down to me, and I was ripping and cutting boards - first particleboard and second vinyl trim - to fit. We actually got into quite the rhythm, and were moving around the house quickly. I was using a cheapo table saw for the ripping, and to set the scene a little bit, the cheap table saws are only about 12 inches tall. I had it sitting on the deck (no guardrail), which is about 12 inches above the other ground. together, that gave me a semi-useable height for the table top (still low, but ok).

Anyway, my buddy calls down a measurement, and I happen to have a piece of particleboard that is close. I only need to rip about 3/4 inch off to fit it. You all know where this is going. Anyway, I was also lazy and set the saw so that the 3/4 was between my blade and the guide, not the correct way with the scrap on the far side of the blade (and I know how to deal with the width of the blade, so don't worry about the end result, it was correct). As I finish my cut, I watch the scrap piece start slowly accelerating, and then SHOOT off the table saw backward. A 4 foot by 3/4 by 3/4 piece of particleboard had just become a missile.

I was not standing behind the saw. My buddy's grand cherokee however, wasn't so lucky. I turn around just in time to see this particleboard missile basically explode and into sawdust as it hit the driver side door of his car (he was up on the ladder so he didn't see it happen). Miraculously, there was no real damage to the GC, as the missile hit the plastic piece on the bottom portion of the door, which was somehow unharmed other than a scuff mark. We made some measurements after as well, and found that the scuff was only about 2 inches lower than the table saw top, but the missile had traveled about 10 feet in the air (you do that math to find the speed of the missile, haha).

How did you learn what you learned?

I learned lug nut torque is important because I walked past my truck one day and noticed this.

I learned the hard way that whenever you re-wire a car, it is a good idea to put a low amp fuse in line with the + battery cable and test all the circuits one at a time. If it blows the fuse, then you have an issue. The lesson involved smoke leaking from a new wire harness.

I recently learned that on a ford 302 engine, the bolt that hold the sheet-metal "spider" in the valley needs specific fasteners. Use of a generic bolt that is a tad long will crush the cam bearing. Was lucky to have a supervisor watching when I did my first 302 build. He let me start threading the wrong bolt in before he mentioned it. 

I learned a long time ago that there is nothing you cant do in regards to automotive technology if you are willing to do the job twice.

I learned many years ago that even when someone holds the red jumper cable in his hand and says "positive", doesn't mean he knows which battery terminal actually is positive and that a backwards jump is worse than no jump at all.  Trust, but verify.

I learned that a flannel shirt isnt very flame retardent at all, and once it gets started it burns fairly energetically. I learned this by standing in the path of the sparks when using a cutoff wheel.

I learned that if info doesnt sound right in a service manual get it verified. I thought the piston rod bearing cap torque seemed low but i went with it. Then i got a rod knock. After checking another source it turns out it was the torque spec i used, but the first manual failed to mention the additional 90 degrees after.

I learned that the person on the phone and the person at the door do not communicate.  If you want in, or want your stuff in, you'd better talk to the feet on the street NOT someone in customer service.  

To the OP; I read a carpentry book by Tom Silva (maybe it was Norm Abram, one of the This old house guys) anyway one thing that stuck with me was "Measure twice, cut once is a good rule. A better rule is don't measure if you don't have to." Why run the risk of screwing up a measurement when you could by with holding a board in place and marking it. Its been quite helpful to me.

When I was four years old, I learned not to unplug a vacuum cleaner by grabbing the cord about ten feet out and snapping it away from the wall. The shock that went through my arm and out my legs comes back to me every time I unplug a vacuum, even 46 years later.

I learned to never light a cutting torch with a lighter. I’m still lucky I have my left thumb intact after using said lighter and left my hand in the path of the flame....

I learned a lot in the four years of Machine shop I took in high school. 

I learned a lot by taking a few Fine Woodworking classes through our community college.  

My dad and my friends dads taught me a lot - lots of auto mechanics. Are you right handed?  Why is the wrench on your left hand?  

Although I’ve never blown up a grinding wheel I was taught in Metals I to stand to the side of the grinder when turning it on - I think we watched s movie about “The Day the Grinding Wheel blew Up”.  

I learned where the sweet spot in the clutch pedal was by driving my dad's forklift around an empty gravel parking lot.

Unfortunately, I learned how dealership service departments work by working in one.

In reply to KyAllroad (Jeremy) :

While cleaning out my dads garage we found a cheesy pair of Jumper cables.  They were separate cables and each one had a red end and a black end.   Into the garbage can....

when I was a young boy (about 6ish) I learned that you NEVER pick up a torch by the end the flame comes out of. My father was using a fan  nozzle on a small map gas torch and I thought I would be helpful and hand it to him.

About 10 years ago, I was helping one of my troops work on a truck I had previously sold him. We were replacing the brake master cylinder that had failed. The new master cylinder did not come with a new reservoir, so we had to pop the old one off. We got the old master cylinder off and I put it in the bench vise. I had to use a 2 foot pry bar to pry the reservoir off. It took a lot more effort to separate the two. When it did separate, it did suddenly and I got hit right in the face with about 1/2 cup of nasty brake fluid.

I just stood there in shock, eyes wide open. The auto shop manager came running over to make sure I was OK. Amazingly, my contacts shielded my eyeballs from the brake fluid. They started to shrivel up and get nasty. I was scared to death I had just blinded myself.

I learned that day TO ALWAYS WEAR EYE PROTECTION. You may not think you need them for a task but I always wear mine whenever I do anything under the hood.

Too many times I have re-learned the lesson "if you have to use a lot of force, you're probably doing it wrong". 

I think age helps with this. Now, I have handy things like another car to drive, so most things aren't under a specific time constraint. MUCH better to just walk away / soak the thin in PB Blaster / look at other options. 

My last one was the oil drain plug on my '06 MX5. hideously boogered up.  (Minor confession: I paid my mechanic to remove it; he later told me he had to pound a 12-point socket on what was left of it & use air tools to remove it).

So maybe a corrolary - "occasionally it IS worth paying someone else to do the job for you"

I cant wait to see all of these lessons compiled in a single thread.....

I learned to remove spark plug wires ONE AT A TIME. At 19 I was not mechanically inclined but thought I was. I had a '76 Triumph Spitfire that needed a tune up, seemed simple enough. Went to the local parts store and bought; plugs, wires, dizzy, rotor, etc. I started the project around 4 PM, (a side note, I was starting a new job in the morning at 7 AM). I enthusiastically ripped all the wires off, pulled all the plugs and removed all the other parts. Started with replacing the rotor and distributor, then replaced the plugs, this is pretty easy.... then I got to the wires, huuummm, this one seems like it is the correct length for this plug, this one fits here, heh I got this. About 6 PM I declare victory, jump behind the wheel and it fires right up, NICE! Get up the next morning, get in the car, it starts right up, go to pull out onto the street and it just barely runs, anything above idle it just bucks and coughs, no power, WTF? I limp it to work managing to get there just in time. Figure after work I'll see if I can find out what the problem is. During the day the guy I am working with and I get talking about how the car isn't running right. He is a bit more of a mechanic than I and mentions it sounds like the plug wires didn't get put where they belong and his brother in law happens to have a Spitfire and his house is on the way back to the shop and we can stop and get the proper firing order. Armed with the correct order the car ran much better. Unfortunately I have learned most of the hard/wrong ways to fix cars over the years. I have finally gotten to the point where I know when to ask for help instead of blindly forging ahead.