When you integrate a constant and get the right answer, but then you remember you're supposed to evaluate it at the bounds. The integral of π from 0 to π is just π², which gives you π²/2. But wait—evaluate at the bounds and you get... π²/2 - 0 = π²/2. The same answer. The horror is that you did all that work setting up the integral notation, writing out the bounds, doing the evaluation ritual... only to realize you were just multiplying π by π the entire time. Could've saved yourself 30 seconds and a line of work. Every math student has been here, staring at their paper, wondering why they're like this.