How to troubleshoot multiple apps on Mountain Lion all missing toolbar icons? My machine awoke from sleep, and third party apps as well as Finder, Preview and Safari's toolbar only shows the blank buttons similar to the image below.

After restart the machine, the problem persists. I also tried using Disk Utility to verify / fix the permissions on the Apple installed software to no avail.

How can I troubleshoot or fix this?
Reboot in safe mode (hold Shift on start), login to your account and restart. I don't know what's the reason for icons disappearing but the above fixes it.