If an interrupt happens at just the right time, or actually just the Wrong time, it can cause the EEPROM write operation to fail.

But it's not always necessary to turn off the interrupts.

What I do, is write the value to EEPROM, then read it back and compare it to what it was writing. If they're not the same, go back and try to write it again.

After a certain number of tries (I use 10, but it never takes that much), if it still hasn't written it properly, then turn off the interrupts and write it one last time.

It takes a little extra code, and some extra time, but it doesn't mess with the interrupts.

HTH,