What happens if you write >10000 times to the flash memory of an ATmega?

What will happen?

it may not pass parity check. so uploading code wouldn't be possible.

with that said, I have never had one flash-based mcu failure due to endurance. that's countless mcus since as far as I can remember.

to give you some sense of design headroom, I wrote-to and then read-back from eeprom on a pic over several hours @ 10ms per read/write, without a failure. that eeprom is rated 1M in endurance.

so unless you get some weird parts or you write to it repeatedly over a long period of time, flash failure is unlikely a practical issue.