Hello List!
I managed to get a "reiser4 panicked cowardly" error today. It was more 
by accident than on purpose. Anyway here is what i did:
- I created a partition /dev/sda1 with a size of 1009 cylinders
- mkfs.reiser4 /dev/sda1; mount; umount (everything ok so far)
- I deleted the partition and recreated it with a size of 500 cylinders
- I mounted it again -> it showed the old size (I expected the mount to 
fail, reiser3 worked that way)
- I copied some data to it. As soon as reiser4 wanted to access outside 
the partition i got "reiser4 panicked cowardly: Filesystem error occured"
- After this i wasn't able to unmount the partition or to shutdown the 
machine. I had to do the magic sysreq key trick.

The full log from /var/log/syslog is attached

ps: i know this was rather stupid, but IMHO reiser4 shouldn't panic...