For whatever reason, Eclipse does not work well with Klipper, the KDE clipboard manager, in its default settings. The symptoms: Quite often, you copy a piece of text to the clipboard. When you try to paste it, it miraculously disappears and some old piece of clipboard content is pasted instead.
After simply trying to ignore the problem for some time, I searched and found a solution today: You have to disable the »Prevent empty clipboard« setting in Klipper’s configuration menu (which is accessible by right-clicking on the systray icon).
Intuitive? Not to me…
Disabling the mentioned option might introduce some minor glitches to general clipboard usage (sometimes, the clipboard seems to empty itself). As those occur rather infrequently, I have not been able to find out why this happens, or even if this is connected to the configuration changes described in this post.