Not the worst, but probably the only one I can sort of explain & not get into trouble for NDA breach..

Umm.. here it goes.. wrong id returned from db procedure, tried to do something on db with that id and got exception that the id doesn't exist. Instead of checking why the procedure returns nonexistent id, he just wrapped everything in try catch without any logs.. & of course, didn't tell anyone about this.. o.0

I know, I know, code review could have prevented this, but holy fuck..
Guy's cv had more experience than I have now, so at the time, I didn't think I'd have to check every line of code he wrote, especially not for shit like this.