when channels are left open after disconnection, when mirc rejoins the channels, the topic is updated to the latest topic in the channel, but if a topic isnt set, the previous topic remains. (ie. joining an empty channel, setting a topic, disconnect, reconnect, rejoin the channel and the topic remains)

this can be very confusing if you dont know which channels still have topics set (eg, after netsplits)

I think it should be OK if you type /topic #Chan to check it directly in your chat window. But I'm not sure I got exactly what you said...