Yep, looks like one of the syncs broke the descent code. When you descend it says, "we're heading for this ground co-ord" but then the other bit of the code goes "what, your not on the ground, how can you have a ground co-ord! Break!". Also means Clarr's unit tests didn't catch it to avoid it when syncing. SO we know the problem, and the coders will be looking at it to fix it ASAP.