Hersheypark's Wildcat is notorious for this. I know a few people who can fit into the seat with no problem and can get the lap bar closed with on problem at all, but can't ride because the seat belt is too short. And these are people who can ride all the other coasters there.
Personally, I have never had the problem (even being a bit on the "chunky side"), however I have been visiting the park with people who have, and it is very embarrassing for them, and it (rightfully so) brings them down for the rest of the day.
The "OTSR's" on B&Mers are a different story. I've not personally known anyone who could not fit into the "big boy" seat, however I do know some people who can not do a front seat ride on them (which is the best way to experience an inverted coaster).
The seat belt thing just seems to be a fluke… other belts fit, even the more tight fitting Lightning Racer. However the Wildcat just ha short belts… no two way about it.
As for the B&Mers, someone said that it is because they are designed by Europeans and the average European is of a smaller stature than most Americans. Any truth to this? Or is this just talk?
-------------
"I wasn't always this cynical, but then I started kindergarden..."