I use my own custom highlight script I wrote which employs the use of /flash.
Someone had highlighted me at the time I was looking at another non-mIRC window, at which point I selected Do-Not-Disturb from right clicking the tray icon menu.
At this point, two things started to happen.
1. CPU usage for Explorer.exe (Im on Windows 8.1)
2. mIRC had completely frozen forcing me to kill the process
At first I thought it might be from /tip or /beep but those was fine.
When I remembered that my script employs the use of /flash I decided to test that out by setting a timer (/timerflash -o 1 5 /flash) and going to another non-mIRC window, then right clicking the tray icon and selecting DND, the same happened again.