Dumbler
A really bad or dumb bumbler. Basically, one who makes too many mistakes too often without realizing it until later.
The guy who just moved in with me was definitely a dumbler at first but at least it seems he's getting used to things now. So he's good.
Dumbler
Someone who royally F's up everything they do.
That guy who keeps giving away the answer unintentionally is such a dumbler.