Some chips only support a limited number of writes to a sector until it is erased because every write - even if only affecting some bytes on the page - slowly weakens the charges of previously written data.

I think I saw a reference that that the flash memory of the LPC21xx or if it was LPC23xx ARM chips supported no more than 10 writes to the same sector before requiring the information to be read out, the sector erased and the information rewritten again. If more writes where performed to the same sector, then you wouldn't get the expected retention time of the information.

This is a bit of a problem, since the information is often missing in the datasheets. Or maybe it is just I who have a problem finding it.