Hmmm I've done so many dumb ass things its hard to pick out just one!
Here's a tale from my private pilot days. I was going out for a pleasure flight one day, feeling invincible because I had 100 hours in my logbook... so experienced! Anyway, a student buddy of mine asked if he could come along and sit in the left seat to fly a little once we were in the air. I said "Sure!" How hard could it be to fly from the other seat? that was my first life threatening error of the day.
As I preflighted the old Cessna 172B, the airport's mechanic asked me if I could run a part over to a neighboring field, just a six minute flight away. I said "no problem!" As we taxied to the end of the 2500' by 30' wide strip, I looked at the sky and noted that to the northwest, the sky seemed a little on the dark side. No matter, I thought. Six minutes there and six back, what could possibly go wrong? I never thought to get a weather briefing for such a short flight (second life threatening error). With no worries, I took off into the hazy sky (third life threatening error).
The flight to the neighboring airfield went smoothly, aside from the darkening skies to the northwest and the haze which lowered the prevailing visibility to around 4 miles. With the other fella in the left seat and at the controls, we played around a little and then I retook the controls for the landing. I had some difficulty, as my hands didn't seem to share my view that right seat flying should be an easy transition for an "experienced" hundred hour pilot.
As we lined up on the runway for the return to the aircraft's home base, I became increasingly concerned with the darkening skies. Instead of going back to the ramp and calling flight service to get a better handle on the situation, I actually said to my trusting passenger "we better hurry, I think there is weather coming!" And thus I made the fourth in the series of life endangering errors that day.
By the time we reached the field just six minutes later the view outside the aircraft had become vastly different. The sky was purple, rain showers that where impossible to see through where touching the northern field boundery and the trees below were bend in seemingly impossible directions by the wildly shifting gust front winds. And thus, I made what should been the last mistake of the day, indeed of my entire life. I didn't run south. I lined up and attempted the landing on the 35' wide runway. From the right seat.
Believe it or not, I didn't crash, although we very nearly wrecked the airplane trying to get it to a tie down spot. I don't know what the wind speed was on landing, but it took six rather large men to hold the aircraft on the ground as we tied it down. Why did I commit all of these errors? Pride mostly. Ego, stupidity and ignorence of the fact that haze can very effectivley hide the presence of convective weather at low altitudes also played a role. I learned allot though. I learned that there is no subtitute for careful preflight planning, even for the shortest flight. I learned that you never shoud try something for the first time without a qualified CFI sitting in the other seat. But most of all, I learned that I should always listen to that little voice inside my head that was quietly telling me the whole time that something wasn't right.
These are lessons that I still take with me every time I get on the flight deck of an airliner all these years later. Hope y'all enjoyed it, and sorry about the length!