I was in the same channel (ie. same channel name) at two different IRC networks.
There was activity in one of the channels - and in the other channel (at the other server) there was no activity.
Then when I was disconnected from both servers, mIRC started rewriting the logfiles. When it was done (well, it actually didn't get done because it ended up with 0 kb free space on C:) I got one log file named #foo.IRCnet.log and one log file named #foo.Quakenet.log.
Now here's the strange thing: #foo.Quakenet.log was several MB's big, and contained the activity from IRCnet. And as I said, there was no activity in this other channel (at Quakenet).

I've received reports of this a few times, have never been able to reproduce it though. Can you tell me your logging dialog settings?

Now this happened again and went on for over an hour before I gave up and killled it.

My logging settings:
Automatically log: Both
Timestamp logs: On
Strip codes: On
Lock log files: On
Trim log files to: Off
Include network: On
Make folder: Off
Date Filenames: -
Folder: C:\mIRC\logs\