This may be a sort of lame bug report as it's quite rarely relevant (and as far as I know could be a browser issue or something), but I thought I'd pass it along.
If I go to the "Users" page on MO and search for a string in the search box, I get the list of users whose username contains that string. If there's more than one page of such users, the numbered list of pages of such users (the one with the "next arrow," etc.) changes to reflect this diminished list. These buttons don't work all for me - they basically don't reflect the search. For example, if I hit "next" is forgets that I've searched and takes me to the second page of the complete list of users.
I suspect and will not prove that your bug has been reported before on meta, that it won't be fixed because it is part of the software that is not supported, and that it won't be an issue if and when MathOverflow moves to a new platform.
In the meantime, the Tips and Tricks page tells you how to use a Google search on just the User pages. You might try it to see if it helps.
Gerhard "Email Me About System Design" Paseman, 2011.07.02