The horrid "There may be problems with the work item type definition" error

If you've changed any work item templates in TFS, you most likely have run into the horrid There may be problems with the work item type definition error. We've run into this error a number of times and it's been caused by a number of things (e.g. local cache out of sync with server, having a group the same name as a state, looking at the system too long ;), etc.).

Anyway, today's horrid There may be problems... error message was caused by having a field REQUIRED in two Team Projects, and not REQUIRED in a third Team Project. When we removed the REQUIRED element from one of the first Team Projects, we got the error in that Team Project. The only way we were able to fix this is first remove the REQUIRED element from the field in question in all Team Projects. Then add it back to the one Team Project that wanted it.

Tracking down this issue was difficult, but since we were only making this one change we knew it had to be related to the field. This is why we make one change and a time and test it out before moving on to the next work item template change. If you make too many in the row and get the error, good luck backing out all the changes to try and figure out which change is the culprit.

While this seemed to fix the issue, we can't seem to reproduce it so unfortunately I don't think I can submit it to Microsoft. Anyway if you run into this situation, try removing all rule elements (e.g. REQUIRED) from the field and then re-add them. That is what seemed to fix this for us.