Just a random error which happens to any player. Althought 0.3z is a way better than buggy 0.3x, there's still some problems with stability. Hope 0.4 will be at least almost bug-free version.
Your system configuration may also cause you to have more crashes than usual. If you think those crashes are too much, try to set your system acceleration a little bit lower. Dunno whether it helps. The problem may also depend on which server you're playing. Some servers causes more crashes than others.
