I cannot exactly reproduce it reliably, but it seems to happen preferably with overlapping windows.
After running mIRC I join 5 servers with about 30 channels; and as I do that most channels appear in the background. One of those that opens first incidentally also has an URL in the topic, which consistently flickers if i hover over it. However, if I switch to that window, the flickering disappears. It won't come back either if I bury that window in the back as it used to be (unless rather randomly, where it does return).
The take-away is probably looking at non-foreground and/or overlapping windows to catch this one...
