I think I may have found a bug in mIRC v6.03 with the 'Flash on:' option that's located in the 'Options' dialog under IRC -> Options.
* How to reproduce this bug:
Enable 'Flash on:' for 'Channel message'.
Now, when someone says something in a channel and your working in another application, mIRC should flash, but it doesn't!
This also goes the same for disabling the 'Flash on:' option. If you try to disable the option, it still flashes anyway.
It seems you have to exit mIRC and restart mIRC for the changes to occur. Shouldn't the changes occur as soon as you click the 'OK' button in the 'Options' dialog without restarting mIRC?