I'm about 99% sure it's a Capsicum annuum pepper. That's exactly what it looks like, anyway. I don't know any other nightshades that look like that other than peppers (the ones I've seen usually look like eggplants or Morelle De Balbis). I agree with jbcreix that it doesn't look like Capsicum chinense. There are a number of other pepper species, too, and although I think it's annuum, it could possibly be one of the others.
I'm guessing one of your bell pepper seeds just bounced into that pot and started to grow. I had that happen with a tomato once (at least a couple meters away from where I was dealing with my seeds; I'm not sure how it got there, but it did).
It's also possible that you were cutting up a pepper and a seed got in the container from that.