Suddenly, pressing Ctl-Shft-C in the Delphi XE2 IDE stopped working. You could press that magic key combination all day and nothing would happen. It took hours to figure out what the problem was.
I had installed update 4 for Delphi XE2, which contained, among other things, something called AQTime. This appears to be some kind of application profiler. Anyway, it was AQTime that was interfering with class completion. A quick trip to Control Panel to uninstall the offending bit of software and everything went back to normal.