Penny Post

I sent a mail without a stamp to a mailing list to which I'm subscribed. I
found out accidentially that the mail stil carried a stamp, although it was
not a stamp for the recipient address of that mail but instead the last
stamp I had generated in a previous mail, to a completely different
recipient.

This is a serious bug in terms of security, as it will tell people to what
other people you wrote a message before, thereby exposing you communication
behaviour.