message received; not in log

I am curious about the following anomaly.

I received one message, which should have appeared in a folder.  But it
appeared in my Inbox.  

Very close examination of this message revealed only that it matched the
criterion for putting it in a certain folder.  Looked just like all other
messages from that mailing list.  The header line which was checked is
identical in all the messages.  Also, I looked in the 'log', and this
message was not logged.  So - it is curious.