Players occasionally float into the floor to be bounced back when the server realises.

http://www.youtube.com/watch?v=9JTg29iY8UA

Above is a video of the issue. It was made by GunfighterJ in the #bukkit IRC channel, but I've experienced this issue myself.

It happens sometimes on spawn. I connect to a server, I spawn on the ground, I start drifting into the floor. The problem is fixed by reconnecting.