I have a laptop that until recently would always crash upon startup. It always had some type of ambiguous message like "An application has temporarily disabled compositing". Hitting alt+shift+f12 would always work to re-enable it, and nothing showed up in any logs. Although not a killer, it was extremely frustrating for my fiancee. I figured out that it is related to her KDE install and application stack on top of fglrx drivers for her Radeon HD Mobility 3400. I have tried several times over the past year to update her to newer drivers in an attempt to fix the problem, but nothing worked.
In the end, the solution was simply to disable KDE's (or KWin's to be specific) step where it checks if compositing is running too slowly. This check was failing mistakenly, and disabling it hasn't caused any issues for me. The specific change I made to disable this compositing check was in the file /home/<user>/.kde/share/config/kwinrc and I added the following lines to the section at the top called [Compositing]: