I believe this is a bug..
https://forums.mirc.com/ubbthreads.php?ubb=showflat&Number=169789&page=1#Post169789

Confirmed.

mIRC 6.21
Windows XP Media Center Edition

Message is sent after pressing the "W" key for around 108 seconds.

Confirmed... 6.21, WinXP SP2
It also occurs when you're pasting large ammounts of text.

I have v6.12... same there too

Well, it always did that since I use mIRC and I don't consider this as a bug...

Why wouldn't it be a bug?

Why would it be?

It sends lines to the server when it's not meant to.

I guess it depends on whether it was intentional for some reason or not. Whether intentional or not, I don't think it should send it if you hold a key down for a long period of time. It may or may not be a bug based on what Khaled meant to happen, but it should be prevented regardless.

When the Windows single-line editbox becomes full it treats that as if a return key has been pressed and clears the editbox. Some versions ago I placed a limit on the number of characters that could be typed into the editbox to prevent the above problem. However the limit also prevented users from pasting multiple lines into a single-line editbox, eg. in the case where they wanted to send several separate lines into a channel or query window. So the limitation was removed to permit this.