I've seen $cb(0) being NULL during $inpaste - in XP Home, under both mIRC 6.16 and 6.21.
The other situation I've found where $cb(0) is NULL happens during adding a large amount of text from certain .html pages, where it can often take several minutes before the text can be pasted. During the time period when nothing can be pasted, the $cb(0) is also undefined.
I had even made a timer to play a sound when $cb(0) changes from nothing into a number, so I would know when I could paste the text.
