I'm not sure exactly what caused this, but it does concern me. The seedlings are supposed to have four cotyledons, and this guy starts off with three. Now one is shriveled up - who knows how long it's going to last. Luckily there's several more in the queue.
I'm hoping that this was due to my keeping it in a glass that was taller than the seed leaves. Maybe there wasn't enough air circulation around the leaves, which started killing it off. I removed it from the glass, and now have it out in the open, receiving a tiny bit more sunlight, though not yet direct -- I'm waiting for the secondary leaves to come in before I give this guy direct sunlight.