[lolc]

I'd assume that parity is checked on access, which may give enough time to flip more than one bit before it's detected.

[Rebellos]

ECC memory controller performs memory scrubbing periodically, in the background, during which it checks parity and corrects any bitflips. Otherwise ECC would not work nearly as well as it does.

[Dylan16807]

Parity isn't checked during every single row refresh?

[cesarb]

AFAIK, row refresh is done within each memory chip, while the ECC bits are normally on a separate chip (for instance, where a non-ECC module has 8 chips, an ECC modules has 9 chips), so ECC scrubbing has to be done in the memory controller.