A bungler is someone who is incompetent and frequently makes mistakes. Bunglers goof things up.
To bungle is to make a mess of something, and a bungler is someone who typically screws things up. A thief who always gets caught is a bungler. An actor who constantly forgets lines is a bungler. A politician who makes a lot of gaffes and mistakes is a bungler. If you want a job done right, don't ask a bungler to do it.