Ok, this one is really obscure:
1. Create user variable foo on computer 1
2. Open and save on computer 2
3. Rename project on computer 1. Add watch on variable foo.
4. Open on computer 2. Variable foo no longer exists.
5. Recreate foo on computer 2. Attempt to place a watch on it.
FB thinks there is already a watch on it, but it doesn't show in the list. You have to do "delete variable watch" in the variables list.
Told you it was obscure...
Steve