Because I took it by mistake thinking it was something else
I am on a cocktail of meds right now which is why I cocked up, but I don't want to start the penicillin course until after my Big Drinking Night Out on Thursday. I have apparently had tonsillitis for months, which I only found out after some tests, so it needs dealing with, but a few days won't matter.
So, if I'm one tablet short of a full course of penicillin, how much does it matter?