Yes, I also noticed this problem on N800. If you keep hitting rewind button frequently, it sometimes jams (most likely because of some issues with gstreamer usage), but I have never seen crashes yet. In order to use rewind in MPlayer, it is better to do it using single key presses: you press a key, mplayer jumps to some other part of movie and start playing it, you press the key again and so on... If MPlayer got stuck, pressing the middle key on arrowpad twice can help. Hope this helps. And I'll try to investigate and fix this issue later.