Subtracting 1 from a bugzilla bug id gives me a "bug does not exist error". This is odd.
e.g. https://bugzilla.redhat.com/show_bug.cgi?id=268180 fails My previous bug, submitted a few seconds ago has a bug id of 268161, twenty lower.
With our new move to a replicated MySQL server environment, we had to change the increment value to 20. This allows future growth when dealing with master to master server replication. So this would explain what you are seeing. Dave