[MySQL] Sometimes I get empty rows from SELECT query...

Hello! I'm writing a PHP script which accesses a MySQL database. As it turns out, sometimes I get an empty row from a SELECT query which should normally return one or more rows. The same query executed a while later, on unchanged data, returns the rows perfectly fine. No errors are returned, just mysql_fetch_array simply returns nothing. What can be a caouse of this? Perhaps I forgot to put mysql_free_result somewhere, and it causes such effects? Anyone had this problem before? Best regards, Michau.