caracter problem with MySQl

I've been using MySQl for some time now and all was ok. Till lastly i found that, in only some places, caraters like apostrophe(') have been converted to interogation marks(?).
Only thing i retain is that i frequently take fata from and to the test server (using export).
I've been doing tests and the problem hasn't occured again though.
However, can anybody help me on how not to get that problem again?
Thnx in advance