When an options window of mIRC is open (e.g. Options, Address Book, Channels List or any other default window, even the About mIRC window) then the custom Scripts Editor shortcuts stop functioning (Check Brackets, Go To Line and More Files)
Not a very big bug but non-functioning shortcuts make tasks longer! :P
This doesn't appear to be affected by custom dialog windows.

Note: this is of course with mIRC 6.31