Version:           unknown (using KDE 3.1.0 (RC5))
Installed from:    compiled sources
Compiler:          gcc version 3.2
OS:          Linux (i686) release 2.4.19-gentoo-r10

I really like the alt/ctrl + printscr feature, it's a really quick way of getting a snapshot without going through ksnapshot. However, it was working very unreliably for me and, after some testing, here is what I found:

There seems to be an upper limit for how much data that reliably can be pasted through the clipboard. I run my monitor at 1600x1200 and ctrl+printscr seemed less reliable than alt+printscr, but after some testing (and asking in #kde-devel) I realized that it all depends on what the screen looks like at the time of the snapshot. With all windows minimized and a simple desktop background, I can't get ctrl+printscr to misbehave. With a large jpeg showing in a window, alt+printscr fails most of the time.

"most of the time"? Yes! The (from my horizon) really weird thing about this is that the success of the pasting of large amounts of data seems somewhat random. I can do one "copy" and then attempt "paste" several times, and on the fourth attempt it might suddenly decide to work.

To reproduce:
1. High resolution, busy screen (not easily compressed as a picture) + use ctrl/alt+printscr. 
2. Attempt "paste" (to the desktop, see #47418)
--> (most of the time) this fails with a "the clipboard is empty" message. 

(Keep trying, it might finally work.. ;) )