Description:
Suppose the following table:

CREATE TABLE `foo` (
  `foo_id` int(11) NOT NULL auto_increment,
  `bar` int(10) default NULL,
  `baz` varchar(32) NOT NULL,
  PRIMARY KEY  (`foo_id`)
) ENGINE=InnoDB;

Where the column bar contains unix timestamps and the column baz contains normal text. A
query that orders by 
IF(bar IS NULL, '-', FROM_UNIXTIME(bar, '%d-%m-%Y'))
will not (correctly) order the result, whereas ordering by bar is not a problem.

How to repeat:
Create the above table, enter some test data like:
1 | 1112344747 | Here's some text
2 | 1112613381 | Text of some kind
3 | 1112708785 | Rather strange table

And run the following queries:
SELECT foo_id, IF(bar IS NULL, '-', FROM_UNIXTIME(bar, '%d-%m-%Y')) AS qux, baz FROM foo
ORDER BY qux ASC;
SELECT foo_id, IF(bar IS NULL, '-', FROM_UNIXTIME(bar, '%d-%m-%Y')) AS qux, baz FROM foo
ORDER BY qux DESC;

You'll get the same result twice

A patch for this bug has been committed. After review, it may
be pushed to the relevant source trees for release in the next
version. You can access the patch from:

  http://lists.mysql.com/internals/24852

Integer overflow results in wrong calculated sortlength.

Fixed in 5.0.7, changeset 1.1958

Noted in 5.0.7 changelog.