I experienced this on phpBB.com only, as this is the only board, that's using phpBB3, and i'm using the '<' character in my username. When doing an egosearch, the pagination links to the username, like this:

http://www.phpbb.com/community/search.php?st=0&sk=t&sd=d&sr=topics&author=eviL%26lt%3B3&start=25

The problem is, that my username contains html, wich - with htmlspecialchars applied to the username - results in < being converted to %26lt%.

Possible solution:
In search.php, find:

	$u_search .= ($author) ? '&author=' . urlencode($author) : '';

Replace:

	$u_search .= ($author) ? '&author=' . urlencode(htmlspecialchars_decode($author)) : '';