That's not, strictly speaking, a mIRC bug. What's happened is that Windows itself has detected that mIRC has gone into a not-responding state, and when that happens with any application, the app doesn't respond to any keypresses. Depending on what caused the app to go into the not-responding state, sometimes waiting for the app to respond will work, and you can regain control; other times, you'll have to terminate the app.
It's possible, but not determinable from the video, that a bug in mIRC is what caused mIRC to go not-responding; more likely, your script probably looped fast enough to prevent the Windows message-passing mechanism from "getting a word in edgewise", and Windows detected that as "mIRC has stopped responding".
The Windows detection mechanism involved actually existed as far back as XP, but has become more sensitive to "race" conditions over time. I suspect that your test would be detected in Windows 8.1 and 8, very likely in 7, possibly in Vista, and likely NOT in XP.
Edited by FreeTrav (21/05/15 11:54 PM)