[TigerC10]

It could be a database backup, do you host this website yourself or do you have a hosting provider? Because I've known hosting providers to lose servers and restore backups only to have a hitch in the backup or in the restoration of the backup.

EDIT:
Nevermind, I figured this one out. Instead of using a standard "M" in the username, this person used the greek letter Mu html character code "Μ" or "Μ". This allows for a completly new user with the name that looks just like someone else's since the character "Mu" is not the same as M.

Here's a list of some other greek symbols that can be used for registration fake outs:
http://www.w3schools.com/tags/ref_symbols.asp

Alpha, Beta, Epsilon, Zeta, Eta, Iota, Kappa, Mu, Nu, Omicron, Rho, Tau, Upsilon, Chi


To fix it, add these to your illegal user names

AdminCP -> vBulletin Options -> User Registration Options -> Illegal User Names

Α Β Ε Ζ Η Ι Κ Μ Ν Ο Ρ Τ Υ Χ ν ο

Α Β Ε Ζ Η Ι Κ Μ Ν Ο Ρ Τ Υ Χ ν ο

Or if you really want to be strict about it, just add a singular semicolin like ';' to the illegal name list.