The Aptitude Test / Oskar Gets a Job - S3-E9
Plot hole: Mr. Simmons asked that each student put their name on the test folders, but the folders already had names on them when he handed them out.
The Aptitude Test / Oskar Gets a Job - S3-E9
Plot hole: When Eugene dropped the tests, just about all of them fell out of their folders, but apparently only Harold and Helga's were switched.
Suggested correction: We don't know that definitively. Those were just the only two caught as the teacher noticed their tests were very off from who they were as students. He didn't appear to check any of the others for mix ups.