OK, as far as having seen it happen yes. As far as the theory, I can only guess and babble with the little knowledge I have. As far as I can guess the reason that flash has a specified retention (10 years) is due to the fact that the bit is stored in a capacitor with very little, but some, leakage. If you do not charge the capacitor enough it will not last as long before the cap is discharged enough to misread.

Again re theory it is babble, maybe someone has the Physics down and can make it specific.

Erik

Just for the sake of completeness there are recorded cases of ROM fuses growing back when not blown hard enough.