I have two "incidents" that are pretty close to a tie. On probably my 2nd or 3rd batch, I was unaware of the danger of boil-overs. I poured in my extract and hops, then went into the next room to watch some TV while I waited. A little while later I smelled something cooking, so I came back into the kitchen to discover wort all over my burners, some of which was burnt to a crisp. Talk about a pain in the ass to clean up! Luckily I cleaned it all before SWMBO came home.
The second incident was a clogged airlock explosion similar to MLynchLtd. I ferment in a kitchen closet and didn't use blow off tubes at the time. At 4 am I got up to go to the bathroom, and decided to check to see how the batch I had brewed up at 7 pm was coming along. Another disaster. Oatmeal stout on the ceiling, floor, other carboys, boxes, everything. It must have just happened because only a small pool of wort had formed at the base of the carboy. I picked up the carboy and placed it in my brew kettle to deal with in the morning. Its a good thing I did, because by morning there was a couple inches of wort in the kettle and wort was still slowly flowing out of the carboy. If that had all been on the rug it would have been really bad, but the cleanup turned out to be easier than it looked. The gunk on the walls and ceiling came off with a sponge and a wet towel took care of most of the floor. SWMBO found the incident more amusing than anything. Its hard to say which was worse, the boil-over or the blow-off. Lessons learned in both cases though.