make a complete ass of oneself(Dear old Richard has made a complete ass of himself. He opened his big mouth in front of Aunt Emma when he should have kept it shut.ART Vancouver)
he really screw up(Thinking that she was a qualified interpreter, they gave her a job, but she really screwed up – Её пригласили как квалифицированного переводчика, а она села в лужу Alex_Odeychuk)
have several seats(When somebody says/does something so unbelievably dumb that they need to just sit down, shut up, and not do anything else; when the stupid is so strong that just one seat will not do. Sebastijana)