Das hab ich andere Stelle irgendwo auch schonmal in nem Buch oder Datenblatt gelesen (Erste Stelle im Eprom löscht sich).
Auf die schnelle find ich nur das: (Datasheet Atmega8, Seite 21)

During periods of low VCC, the EEPROM data can be corrupted because the supply voltage
is too low for the CPU and the EEPROM to operate properly. These issues are the
same as for board level systems using EEPROM, and the same design solutions should
be applied.
An EEPROM data corruption can be caused by two situations when the voltage is too
low. First, a regular write sequence to the EEPROM requires a minimum voltage to
operate correctly. Second, the CPU itself can execute instructions incorrectly, if the supply
voltage is too low.
EEPROM data corruption can easily be avoided by fol lowing this design
recommendation:
Keep the AVR RESET active (low) during periods of insufficient power supply voltage.
This can be done by enabling the internal Brown-out Detector (BOD). If the
detection level of the internal BOD does not match the needed detection level, an
external low VCC Reset Protection circuit can be used. If a reset occurs while a write
operation is in progress, the write operation will be completed provided that the
power supply voltage is sufficient.