AFAIK, 255 is a table field limit. I believe the limit over the lifetime of a form is 754, so if you put 250 on, removed them all, recreated another 250 and did this until you exceeded the limit you would have this problem. I doubt you have done anything like that. Is there an error message and number? You might be misinterpreting the problem message.
It may only be that the form is corrupt. You might be able to create a new blank one and copy/paste the controls from the problem form to the new one, or try a compact/repair on the db.
Do you have Name Autocorrect turned on?
EDIT - How many bound controls on the form? How many in total?
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.