To reproduce the problem do the following:

1) Read a mysql database table with user names and a double index (UID, server - both numeric) having about 280 entries:

SELECT UID,Server,Username FROM Users ORDER BY UID,Server

2) Preparing the array: $userlist = array();

3) Filling the array:

$result = mysql_query("SELECT UID,Server,Username FROM Users ORDER BY UID,Server");

if ($result)
{
   while ($q = get_object($result))
   {
       $userlist[$q->UID."-".$q->Server] = $q->Username." (".$q->Server.")";
       
       // Debug output just to see that filling happens in the right alphabetical order of user names
       echo $userlist[$q->UID."-".$q->Server]."\n";
   }
}

4) Reading the array:

foreach ($userlist as $key => $value)
{
   echo $key." = ".$value."\n";
}

you suddenly notice that the array entry no. 256 and 257 appear at the wrong place (at position count($userlist)-256) - the internal sort order of the array has changed!

I also tried this with array index names in the form:

[$q->UID.":".$q->Server]

and

[$q->UID.".".$q->Server]

Both with the same wrong result ... arrays read out don't have the same order as they were filled any more if more than 256 entries are present.