Yep, this happens, if you don't notice them, you fall and you're stuck down there for the rest of the game if you live the fall. I'm guessing where the arena was placed, isn't a snow biome, thus, the snow melts. I could be completely wrong, but it could be a possibility.

 

imo, we should get a death match plugin for HG. Like when 3 people are left, it gives all people 60 seconds, then it tp's them to spawn to fight it out. I have seen this on another server and it's awesome. It also doesn't allow you too leave the spawn area or it kills you. This would make the games go so much faster and more convienient.