Actually, this is more a mistake of teaching, but "sin" sounds so much racier, doesn't it?
Here's my mistake of the semester, so far: I gave my students an assignment that's good in many ways, but that I'm not quite sure how to grade.
It's not a huge assignment, but it's a bit complex to think how to grade. So I've been procrastinating about grading, which has made it even worse, of course. (Procrastination about grading is my prime teaching mistake, I think.)
I really, really need to just sit down with this and grade.
Hmmm, I wonder if I get a radio for the front part of my house... I should look at radios! And maybe go out to buy one! Yeah... because that will be a useful strategy, right?