I subbed in a business/computer class the other day.
Computer classes might just be the single most boring class to sub for, especially when the kids are good. All day the kids sat silently and typed away.
In fact, this may have been the most interesting part of the day...
Student: "Hey, is this vocab sheet double-sided?"
Mr. C: *picks up his copy and flips it over, showing a blank side* "Nope."
Student: "Oh, I guess I could have done that."