If you have a subscreen with a total of 128 or more subscreen elements (put simply, if you see a subscreen element with ID 127) and then you save your quest and reload it, it'll say "Bad token!" or something while loading the quest and all your subscreen data will be reset to default (at least... the custom stuff from the point where the overloaded subscreen is will be deleted...).
You can salvage some of the subscreen by trying to make a new subscreen in the place where the old one was, based off the "Zelda 1 (No Magic)" subscreen. Clearly the subscreen data was still in memory, it's just the header became unreadable.
This is really aggravating me because I'm trying to work on a special 'life gauge' that uses about 90 subscreen elements alone - and the background, done in 2x2 frames, takes up 20 more. The 'magic gauge' takes up 8, barely leaving me any room for anything else while this bug is still in.
I dunno if this matters, btw, but I was doing this on a Passive Subscreen...