Edit: Here's another one for the books. Giving an improvement or wonder a Bombard defense value means as we all know that this improvement/wonder can't be built in cities larger than size level 1 (in epic C3C size 7 or more). But any other effects of such an improvement/wonder will still be working in a city that's built it and then grown beyond size 6.
(E.g. if I for some reason would give Walls a 25% increase on production and give 1 happy face, it will continue to do so when the city grows larger than size 6. If I should disband/sell it I'll lose the shield increase and the happy face, and I couldn't build it again until my city is back on size 6 or less.)
Thast actually makes a twisted amount of r/w sense. In Europe certainly, plenty of original walls have been engulfed by the city expanding beyond them; either their stone was recycled and/or they become part(s) of other structures.
-Oz