I realise I'm digging up a dead dog, but I just want to confirm this happens to me _all the time_. And no, I am **NOT** hitting Insert.
There is obviously some sort-of bizarre input problem with mIRC which causes this to happen. I've seen it happen both in text input boxes (for channels/queries/etc.), and in the script editor.
There is a POSSIBLE explanation for this bug listed in another thread (search for "overtype mode"), described as follows:
This is also a bug in mIRC's new handling of Multibyte characters, and should be reported to the Bug Reports section of this forum (if it hasn't been already).
In the mean time, you can reverse this change to how it was in v6.12, by going to ALT+O > IRC > Messages and unchecking [ ] Multibyte editbox at the bottom. You may also uncheck all four of these options at the bottom to completely disable any special text handling.
I myself have Multibyte editbox enabled, as well as SJIS and many other dual-byte options (since I occasionally use Japanese). I'll have to try turning all of these options off to see if that solves the problem.
So you're not alone, Pomax. We aren't hitting Insert -- this is a software bug, folks.