I wouldn't call this a disaster, but: Coding a somewhat complex embedded application entirely in assembler when it should have been done in C from the start.

I wouldn't call this a disaster, but: Coding a somewhat complex embedded application entirely in assembler when it should have been done in C from the start. I knew better, but I got going on the project in assembler and didn't stop. At first maintaining and expanding functionality was not too hard. As time went by it became harder and harder. The fix was to stop everything about a couple of years after the product was already shipping and take three months to re-write it in C. After that adding feature requests and improving functionality was an absolute breeze.