I think we are all interested to hear of the experiences that others have learned from, so in that spirit here's a couple of mistakes I've recently made.
My water pump had started dripping occasionally from the inner seal out of the vent holes on the side of the pump casting. I purchased a new pump, but the leak stopped so I didn't change it, as it moved down the priority list. Yesterday we returned from a 200 mile trip and when I opened the hood, the complete engine compartment was coated in a dirty grey spray of dried coolant mixed with a little ATF from my leaky P/S pump. My formally nice clean engine looks like I just crossed the Amazon with a canoe paddle.
Second lesson:
We live near Ottawa and enjoy travelling through the Gatineau Hills in search of new vistas. Our map showed a secondary road that cut across a large expanse of mountainous terrain and joined with another highway. As the road became progressively narrower and rougher and steeper, I quizzed my wife the map reader "Are you sure this is the right road". "It has to be" she said. There weren't many places to turn around and she was sure we would get there, but I was loosing confidence as the climbs were becoming rapidly more rutted and there was no sign of humanity. We finally reached a summit that was ridiculously steep and there sat a large earth mover next to a mountain of sawlogs on either side of the road and skidder ruts deep enough to swallow a volkswagen. The view around the corner showed worse ruts and tree limbs all over the "road".
We managed to get turned around and drove the 20 miles back to paved roads with me shaking my head and wondering how we ever got so lost. When we got home, we checked three other maps, and none of them showed a road where we had ventured.
I'm adding a chain saw and a tent to my emergency road kit
Ricardo