Some months ago, one of my oft-used keyboard shortcuts in an OS X application broke: Command-/ in IntelliJ IDEA, which is used to comment out lines of code. I am constantly installing new applications on OS X and had recently upgraded IDEA, so tracking down the offender was no small task.
I started out by reviewing the OS X Keyboard Preferences Pane, but there was no keyboard shortcut registered for Command-/. IDEA didn’t show any conflicting mappings either. So I lived without Command-/. It was a bit painful but I couldn’t justify taking any more time tracking it down.
But today, I finally got around to it. I starting pressing Command-/ all over the place, trying to see what would happen. Hitting it in IDEA just causes a beep. Hitting it in Firefox also causes a beep. But finally, I hit it in Finder, and it launched… DEVONthink. I installed DEVONthink when I bought some OS X application bundle a few months ago. I’ve never used it. So I Googled DEVONthink and keyboard shortcuts, but didn’t find anything on Command-/.
And then I discovered Service Scrubber. And sure enough, that naughty little DEVONthink had installed a presumptuous keyboard shortcut for Command-/ associated with one of their OS X Services. I disabled it with Service Scrubber, restarted OS X, and finally, I can use Command-/ in IDEA again.
Ben Galbraith 
Thank you sir! I was suffering the same can’t-comment-in-IDEA problem. I appreciate the fix! I purchased the macheist bundle, and have lived without Command-/ ever since (a couple months now?).
I caught your talk at JavaOne, by the way, and really enjoyed it. Great job!
Regards,
Justin T.