I have a bunch of seedlings started under lights in my basement. All of the seedlings seem happy and healthy, except for this 1. Can someone help me figure out what happened here? What went wrong?

This is a watermelon seedling, by the way. It's being grown in a self-watering tray system. I did top water to set the seeds, but I haven't top watered since.

I've posted pics here: Seed Starting 2012