I have a bunch of these scattered through /var/log/messages on a
couple of servers:

I/O error: dev 08:20, sector 0

The server functions just fine. Anyone have an idea what might be
causing this error?

No. But if the 08:20 is referring to the major:minor device number, perhaps a

ls -l /dev/ | grep "8,"

or similar command may give you a hint.

Alan.