One approach to reduce the costs of usability testing is to facilitate the automatic detection of critical incidents: serious breakdowns in interaction that stand out during software use. This research evaluates the use of undo and erase events as indicators of critical incidents in Google SketchUp (a 3D-modeling application), measuring an indicator’s usefulness by the numbers and types of usability problems discovered. Our evaluation also compares problems identified using undo and erase events to problems identified using the user-reported critical incident technique [CITE]. In a within-subjects experiment with 37 participants, undo and erase episodes together revealed over 80%% of the problems rated as severe, one third of which would not have been discovered by self-report alone. Moreover, problems found by all three techniques were rated as significantly more severe than those identified by only a subset of techniques. These results suggest that undo and erase events will serve as a useful complement to user reported critical incidents for low cost usability evaluation of design-oriented applications like Google SketchUp.