Contacts disappearing


Today, a old msn contact I usually only have on Skype, logged in with their old msn contact twice.
The first time, after it logged out both on Skype and msn, and then back in, the second time only on msn, but after the second time, it just disappeared from my contacts list on msn?
If I open a conversation page with them, they’re considered not in my contacts.
Any help in knowing why it happened?