I don't think this has been reported before, found nothing during my search.
I was copying and pasting an XHTML line to my friend:
<meta content="text/html; charset=iso-8859-1" http-equiv="Content-Type" />
(Indented by two tab spaces.)
Anywaes, when I was checking my debug window I saw.
[13:56:56] -> Om<meta content="text/html; charset=iso-8859-1" http-equiv="Content-Type" />
This seems to work for anything.
[14:03:56!
[14:06:48] -> Omg!
[14:06:52] -> Omega.SE.E:o
One tab, two tabs, three tabs ...
*edit*
This was on mIRC v6.21 incidently.
I've also got my friend to reproduce this
here.