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I set the upper right corner of the screen as a Hot Corner to activate Mission Control. However, some time after setting it, the Hot Corner stops working. Keyboard shortcuts still work.

Going back into System Preferences, changing the Hot Corner to some other action and then back to Mission Control resolves the problem temporarily. But eventually it fails again.

This wasn't always a problem with Lion. Mission Control Hot Corners used to work fine. I'm not certain, but I think the problem started with the 10.7.2 update. Either that, or some other software recently installed on my Mac is causing a conflict.

Any idea on how to resolve the unreliable Hot Corners?

27" iMac Core i3, OS X 10.7.2

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Are you using multiple monitors? In my experience, that makes Hot Corners behave strangely. –  Nathan Greenstein Dec 17 '11 at 0:41
    
Same problem here. Only the top left corner works and the rest just stays unresponsive. –  theAmateurProgrammer Dec 17 '11 at 13:41
    
Nathan, no just the iMac's single integrated monitor. –  Greg R. Dec 19 '11 at 13:22
    
AmateurProgrammer, I can confirm the same behavior. Only the top left works. The other three corners fail. I never noticed the bottom left and right failing because I rarely use them. Top left for Show Desktop, top right for Mission Control are all I use regularly. –  Greg R. Dec 19 '11 at 13:25
    
Possibly related: apple.stackexchange.com/q/46994/10410. –  zpletan Mar 31 '12 at 18:58

7 Answers 7

I had the same problem in Lion. Just go to preferences and assign the corners to something else (or to no action at all) and then change it back, see if it helps. Make sure to accept the new assignment before changing it back to your desired actions.

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2  
As I noted in the question: "Going back into System Preferences, changing the Hot Corner to some other action and then back to Mission Control resolves the problem temporarily. But eventually it fails again." Emphasis on TEMPORARILY. –  Greg R. Apr 12 '12 at 11:48

The problem could be related to a corrupted Dock Preferences file (Hot Corner settings are stored with the Dock preferences). If this is the solution, as a part of it, you will end up having to reset most if not all Dock related settings (scroll bar visibility, scroll bar behavior, auto hide, Dock screen position, icons in the dock, etc.).

Try this:

Open Terminal (Terminal can be found the Utilities folder. To locate the Utilities folder, in Finder, click Go -> Utilities) Type in the following commands:

mv ~/Library/Preferences/com.apple.dock.plist ~/Desktop/

Then:

killall "Dock"

At this point, open System Preferences and reset your Dock preferences and set the hot corners. Hopefully this will work for you.

Command Explanation: The first command moves the existing Dock preferences file to the Desktop to serve as an improptu backup. The second command quits the Dock. Mac OS will instantly reopen the Dock with a fresh preferences file.

If this did not work, you can optionally restore your old preferences file with the following commands:

In Terminal:

rm ~/Library/Preferences/com.apple.dock.plist

mv ~/Desktop/com.apple.dock.plist ~/Library/Preferences/

killall "Dock"

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This keeps the corners operating correctly for a few days, however they've acted up again - seems to be another temporary solution, unless the .plist is getting repeatedly corrupted. –  walker Sep 6 '12 at 12:46
    
Despite my hopes, this doesn't work. Even temporarily. MacBook Pro 5,1 — OS X 10.8.2 Build 12C60 –  Bryson Oct 4 '12 at 22:11

I had the same problem where the hot corners worked for everything but the screensaver, and this appeared after I was forced to reboot when the MacBook Pro locked up.

I tried the "delete the .plist" file solution, even though I'd lose all of my Dock settings, but it didn't fix it.

I used the Disk Utility to repair the permissions on the hard drive (select the disk on the left pane, and click the "Repair permissions" button at the bottom of the right pane), and then rebooted again.

This got my screensaver working again...and I didn't have to sacrifice all of my Dock settings.

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My solution to this was to sleep the display. After waking, the issue is resolved until the next restart. Usually I would do this using a Quicksilver command, but all Quicksilver commands related screen savers, Exposé, et. al are also broken. Since I have a MacBook Pro (5,1) I just shut the lid and reopen it once the entire machine has entered sleep mode (the white light on the front will begin its slow pulse).

It seems that there are a lot of different "solutions", and yet all of them are temporary and not all of them are universally effective. Hopefully this one will work for those for whom the .plist and permission repair workarounds do not (like me).

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Just found the easiest solution! (for me at least)

I'm running on a MacBook Pro 13" together with a second screen and my hot corners would randomly stop working. But when I simply close the lid of the laptop, wait for a sec and then open it back up everything works again.

Hope that works for you!

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That's very little help on a 27" iMac. :-( –  Greg R. Nov 26 '12 at 11:51

I have an external Samsung monitor attached to my MB Pro 17" late 2011. I had the "display port" cable plugged in, and had chosen "mirror monitors."

It occurred to me that the reason "Hot Corners" had failed, is that with the monitor connected to the display port, the MB pro may be "seeing" the monitor, NOT THE MBP's display. When I checked the monitor, it showed is as being connected to the display port, even though it was turned off. I removed the cable and plugged it back in: voila, HC's are working again. I use the monitor for watching download video. In any case, I have left the monitor plugged in, and will carry on to see if I can "induce" the failure. Should I be able to replicate the fault on a regular basis, I will report the result here.

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After my 'hot corners' and gestures on my trackpad stopping working, I just open the app named 'Mission Control' once, then they recover to respond

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