I think I know what triggers this bug.. I removed the ad-banner in the conversation windows myself (with recource hacker) and since then I've been having the same bug. Now I'm using Messenger with the ad-banner in the conversation windows enabled and the bug hasn't accoured yet, weird..
Note: The ad-banner has no further relationship with the custom names. (Just saying this incase some people start thinking Patchou made that bug happen on purpose if you remove that ad-banner, you never know what people will come up with
)
My versions:
Windows Version: Windows XP, 5.01.2600
Messenger Client Version: 7.0.0425
Messenger Plus! Version: 3.40.0112