Shoutbox

Does it also change back at your end to the previous message? If so the cache got corrupted somehow (happens sometimes).

Fix:
Sign out of messenger and exit the program, then
delete the two contact caches (folders) corresponding with your
passport in the following folders:

WinXP:
%userprofile%\contacts
%userprofile%\Local Settings\Application Data\Microsoft\Windows Live Contacts

Vista:
%userprofile%\contacts
%localappdata%\Microsoft\Windows Live Contacts

These folders are hidden so make sure you got show hidden files/folders enabled.

You will notice after that that messenger will take a bit longer to sign in, this is because it has to get everything from the server.