About 40 minutes into an 720x480 m4v stream from my MacBook, the video stopped and I got a "low system memory" warning/error on my Apple TV. After going back to the content menu, choosing the file again, it resumed playing fine. After another 30-40 minutes it happened again.
I've never experienced this error before in the prior two releases. There were also some minor hiccups throughout, but that is pretty much what I'm used to from this beta player.