This is related to Windows/DOS device names. In DOS there are devices named CON, AUX, COM[1-9], LPT[1-9], PRN, and NUL which prevents any filename in DOS or Windows having a file with those names (with or without any file extension).
It's been suggested in the past by Khaled that this is also true for ip, global, shadow - and certainly mIRC does treat those filenames the same way (which is the reason for your bug). However I can't find any documentation explaining why those three additional files are reserved and I can't find any other application that prevents their use. Maybe Khaled or someone else can provide a link to where this is documented?
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