Here's one to stump you all... (no pun intended)
It came up in the spring with other weeds, but this didn't look like normal weed- more like a tree. It's a new house, so lot has been disturbed, so it may have been a dormant seed. Or, I mulched with forest mulch around here, so it may have come from mulch. Someone on another website suggested some type of euphorbia. One of seedlings I pulled up had a fat root on it.