Why selecting text with mouse often doesn't make the text pasteable with mouse mid-button click?

[Yuri]

I notice this again and again: when I select some text in Konsole (KDE4) 
sometimes it doesn't paste with the middle mouse button click.
I also notice the same in chrome, and it's even more likely there.

Why such basic feature fails intermittently?

8.2-STABLE amd64

Yuri