Since CWD-3188, incremental synchronisation with a remote Crowd will fall back to full synchronisation when inconsistencies cause failure. However, some cases don't cause failure. Specifically:

• Synchronisation changes for a locally-deleted user recreates that user.
• Membership changes to a nonexistent group are ignored.

These cases will only happen when the current local state is out of synch with the remote Crowd instance; they imply that there may be other problems. They should be detected so that Crowd falls back to full synchronisation.