Same here. :(
Yeah, I had this too - I had a poke around and found a couple of bugs in categories.php. I've fixed them up and made a patch file - you can grab it here:
A couple of things to note: I've made this patch against the file in v3.1 - I have no idea about other versions. Also, I've done all of about 15 minutes of testing. It seems to work for me. I make no promises, and your mileage may vary.
For those interested, the bug would seem to occur when you try to modify an existing item - adding a new item probably worked fine.
That's about it.