It might happen that there is no comic next week. I can't promise that I'll find the time. I will try my best, though.

Panel 1:
What you think you are writing during the exam:
...and with the lemma from the lecture we directly get the formula. Replace the variables, done. easy.
Panel 2:
What you later fear you might have written:
Oh, the Lemma was maybe a bit different. I definitely switched up a sign in the formula and then I probably miscalculated...
Panel 3:
What you actually wrote:
As sdated in the lecture: For all n it holds that 4 = 5, hence we recieve:
12a - + - + + -(all signs but the last are crossed out) b = c.
With a = 1 and b = 4 it folows the fact that c = 7. q.e.t.