A college class.
Though that was entirely my fault, since I should have done something about it but didn't. Basically, when I first started there, I...couldn't really handle the schedule that had been given to me. Ended up dropping one of my morning classes (since those were the ones I was having problems with at the time), but for some reason I didn't drop another morning class, a design class that I had to take and wasn't doing so well in. Instead, for some utterly stupid reason, like I don't even know why I thought this was a good idea, I decided the best course of action was to just...stop attending that class.
....
Yeah, I ended up retaking it a few semesters later, and actually didn't do too badly in it, but oh god. I was being stupid at the time, pretty sure I even knew I was being stupid, and yeah...don't ever do that, ah ha.