I've run into this occasionally but have not found the exact source. What is interesting--and might indicate that the clipboard gets weird with triplehead resolutions--is that I can almost always do an ALT+PrntScrn to copy/paste a small application window even when a regular PrntScrn copy/paste for the entire desktop fails. When I open the Windows Clipboard Viewer after a failed paste, it will have this message:
ClipBook Viewer cannot display the information in its current format or there is not enough memory to display it. Quit one or more applications to increase available memory, and then try again.
Yet, if I go to the View->Bitmap menu option, the contents of my screenshot will pop up in the viewer like normal, but there is nothing I can do with it. :?