WordNet (r) 3.0 (2006):

slip up
    v 1: make an error; "She slipped up and revealed the name" [syn:
         stumble, slip up, trip up]

Moby Thesaurus II by Grady Ward, 1.0:

20 Moby Thesaurus words for "slip up":
   blunder, bungle, deviate, err, fall into error, foozle, go amiss,
   go astray, go awry, go wrong, goof, lapse, miscalculate, miss,
   miss the mark, slip, stray, stumble, trip, wander