Why single click sometimes generates double click event?

[Yuri]

On my system single click sometimes acts like double click when it 
shouldn't.

There are some apps in which I keep noticing this.
For example, thunderbird often (but not always) opens the message when I 
click on it once. Or, which is especially annoying, it opens the range 
of messages for deletion when I click on the first one, and then on the 
last one with the Shift. Intention is just a selection, and it opens 
them like it was a double click.
And then there is the vnc window, which is very likely to go into the 
maximized state from just one click (kde4 wm).
And in qbittorrent torrents sometimes open on single click.

As I understand, there is no such x11 event as "double clock", apps 
detect double click based on ButtonPress/ButtonRelease x11 events. And 
qt apps detect double clicks in qt framework. kde settings (400ms in my 
case) should be used for this. There is also the line *multiClickTime: 
400 in ~/.Xdefaults which I am not sure affects anything or not.

It looks like the second click follows the first one very promptly when 
such click has never really been made. However 'xev' never shows such 
fenomenon. It also isn't capable to "spy" after another app's mouse 
events for some reason.

Did anybody have such problem?

Yuri