I had a crack in my floorpan where the driver side seat bolts to the floor on the front left corner. The previous owner tried to ghetto rig a fix by cutting big rectangular HOLE there so they could sandwitch the floorpan with washers, and then use a bolt spot welded to the lower washer and a nut to "secure" the seat to the floor. There was no real way to torque down that corner of the seat very tight, and eventually the whole mess ripped right out of the floor one day when I was powershifting into second. When the seat suddenly jerked back it scared the hell out of me, I'm just glad the right side seat frame held, I could have easily ended up in the back seat. I nearly lost control of the car as it was, and could have easily hit the wall had I been at the track, or worse, went into an oncoming lane if I'd been in in traffic and not on a deserted road.
Moral of the story, if you have floorpan issues you should get them fixed immediately... I knew my seat was jury rigged but I never realized just how dangerous it was. I took the car to a bodyshop and had them weld up the hole with a steel plate for about $350 and now my seat is rock solid. I'll be adding subframes soon and am definitely going to get a set with a crossbrace for the rear seat bolts. The Fox floorpans need all the help they can get.