[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.