I was reading about the stripped hanger story (and how he didn't want to pay for a replacement), so I want to hear the worse Home Mechanic mistake. Back in the ancient days where aluminum frames were glued together (no joke), my friend insisted on building his girlfriend's Alan instead of the shop. Well, he put the seatpost in ungreased (I also suspect the seattube needed to be reamed), got it stuck, and ended up separating the seat tube from the lug. He brought it back to the shop for them to resassemble, and never told his girlfriend he destroyed her frame. I actually told her 2 years later, when I realized he never did. Luckily she was not very serious, otherwise that frame would have folded on her.