I've seen this as well and while in some ways it is just strange, it is also pretty serious as it prevents typing many characters/languages at all. I have gotten used to the type-in-notepad, paste-in-mirc workaround, but it is tedious. Bwuser says he has seen it in other software; I haven't though, mIRC is the only thing I have with the input errors.
For an idea of how very random a lot of it seems:
I pasted in mIRC all the Latin letters in the Latin Extended-A range, which contains a lot of the letters needed for major European languages. The great majority of the letters do not get substituted, but I can TYPE none of them. The ones that do get substituted are all the lowercase letters with macrons, lowercase e with caron, lowercase n with acute accent and lowercase n with caron. The capital forms of all these letters do not get substituted; I have tested this with several fonts, all of which have the required pre-composed glyphs, so ... huh?
So really it's two problems, maybe separate:
1. mIRC's editbox doesn't let you type in a lot of Unicode characters, but does allow you to paste them if you type them in notepad, etc. first.
2. mIRC's editbox substitutes certain, specific characters even when it is not needed. notably, this only happens in the editbox .. once you press enter, the text shows up correctly in channel in the same font.
