Futures: never had this happen
I've had many boards with future fin boxes, but just bought a new one, put a set of fiberglass fins in...then couldn't get them back out.
yeah, I've had futures difficult to get out because of sand or water suction (or just a tight box, etc), but never on a brand new board and never this stubborn. These fins would not come out even after about an hour of tugging, wetting and every other non-destructive technique you can imagine.
Finally, I had to resort to drastic measures: a flat head screwdriver as a wedge. Fortunately, the fins I installed are oversized twins, so the front edge goes past the fin box...there was barely enough room to wedge something under that lip. Of course, I almost had to destroy the lip to force the fin out, but I was luckily able to do minimal damage.
Turns out some leftover glass had made it into the fin boxes (neglected by QC) and were creating a wedge effect that wouldn't release the fin (never seen that before).
Moral of the story is always check the inside of brand new Future fin boxes for residual glass/lumps and chip them off...if you put standard fins with these little wedges in there, you will not be able to get the fins out without damaging them, or worse, the board. If the fins seem difficult to get in the new box, you might have a "wedgy".