Originally, this was just to fix FOFs with the Bouncy FOF sector special still being able to bounce players off them even when made to disappear (i.e not have FF_EXISTS; one way of doing this is using linedef type 445 on them). However, I also spotted that Space Countdown probably has the same problem and so I fixed this too. Sector specials should not work on FOFs that do not "exist"!
EDIT: Updated to include fixes for the Each Time trigger specials as well as object-carrying scrollers.