*I had an off-by-one error in a TopCoder problem (I used > instead of >= in a loop) that I didn't catch that cost me $3000 in prize money and a trip to the finals.
*I was working at an observatory on campus and left the huge, Peltier-cooled CCD for the telescope on a table but still plugged into a computer and left for the day. When I came back, I found that someone had tripped over the cable, smashing the CCD on the floor. They then sat the broken CCD next to the computer without a note or anything. $7000 CCD destroyed.
*Another time I was working with an AFM in a basement of the university, and left for the day. It stormed really hard that night, and when I came back the next day the basement had 6 inches of water in it. It turns out that the water had come from a leak directly above the AFM. I guess the AFM didn't like getting a shower in filthy storm water and it cost $20-$30K to replace.
*However, my biggest save was probably more important than all of that combined. Without divulging too many details, I was writing some tests and caught a serious data-loss bug in production before any customers were affected by it. The bug actually made the news: http://www.theregister.co.uk/2...