I found a random seedling in my compost pile and decided to pot it and see how it would grow. It has survived and at first I thought it was a squash or a melon. When it started growing, its first leaves looked something like a fish, with a little tail at the end, squeezing in the middle, then tapering out again (reminds me a little of a dandelion leaf). It's still fuzzy like a squash leaf though. As it grew out, it has stayed rather spindly and little vines are starting to grow out a curl at every leaf node. The leaves are also now very round and kinda spiky.

I remember throwing out alot of canteloupe and butternut squash seeds, but this doesn't look anything like any of those plants which have already been established. Could it be watermelon?