When you are setting up a Display Rule, if you use the GENERATE option, the default setting is 'REPLACE EXISTING'. although this is OK, if you are modifying or adding new Display rules, it is very easy to forget to swap it to 'ADD AT END'.

The Replace Existing will wipe out all your work and there is no UNDO option.

I think there needs to be a warning asking... Are you sure.............. YES/NO