Why selected in terminal text sometimes doesn't get into the clipboard (XA_PRIMARY) and can't be pasted?

[Yuri]

I run 9.1, kde4, nvidia driver. All updated.
Again and again, I see that selected in konsole text doesn't get pasted 
with the middle mouse button click in approximately 1-5% of cases. I 
think the problem happens only in konsole, but I am not positive. It's 
also hard to reproduce, but usually happens sporadically, now and then.

This problem has been bugging me for years, probably for 5 years or 
more. Doesn't seem to change a bit with many system updates over all 
those years.

Anybody sees such problem too?
I think the problem is either in xorg or in kde.

Yuri