I've actually gotten footstooled myself after my fair ended, lol. Pretty awful when that happens, I don't know the exact details on this whole f-air footstool thing. Amazing when you land it, horrible when you attempt it and it backfires like that. If I knew the conditions in which it could backfire, that would help a lot. Am I the one who failed, or did the enemy have a frame advantage after my fair ended (time from my fair ending > the histun from the last peck of the fair that hit the enemy at low %) -> thus leading to them being able to jump before I could?
It'll be exceedingly rare because no one full hops in the neutral, but I've theorycrafted about an "on-stage" fair foostool -> enemy footstool lands -> Falco fastfalls and tech chases or something. Maybe it's only possible in a platform stage like Battlefield lol. Don't know.