1) For my non-flying oops moment, I used to work cleaning airplanes overnight for a major airline. Normally we did this when they pulled the plane into the maintenance hangar, but this night they left the plane at the gate for minor maintenance. When this happened, the maintenance guys would let us borrow their van since it was on the opposite side of the airport from the hangar we worked out of. I had to reposition the van for some reason, and being tired and not thinking things through, I thought I'll just back it around to where we need it. I immediately ran over the chocks and bounced the crap out of that POS van, and nearly backed into the engine nacelle in the process. I got out thinking "That was dumb, but at least nobody saw." It turns out the maintenance supervisor had been watching me from the terminal building. The next day he asked if that was me backing the van up. I hung my head in shame and he said "I don't ever want to see you back the van up in the gate area again."
2) For my flying oops moment, it was weather related. I was out on a training flight with my instructor. This was summer time in the Midwest where storms can pop up out of nowhere. There wasn't even a chance of rain forecast when I was doing the preflight. As tends to happen that time of year, small showers started popping up. As we were flying along, we were discussing how you can see the outflow of wind blowing dust out the bottom of a rain shaft. About that time, I noticed that the building storms were starting to box us in. I made a beeline for the airport and we landed just as a storm started dumping rain on the airport. I remember the instructor saying "We need to stick this landing or our go-around is going to put us into a storm." I got much better at being proactive with pop up storms after that.