While the other answers do answer your question, I just wanted to point out that it is best to provide your own error checking routine (or code) so that your script will show pleasant error messages to users.
something like
$result=@mysql_query($some_query);
if(!$result){
if($debugging==TRUE){
echo($some_query.'<br>'.mysql_error());//shows error in debugging mode
}
else{
log_error()// error logging function
die( 'there was a problem with your request the problem has been logged and we are working on it');//or something like that
}
}
//no error
more code