Misbehaving applications, moving files between users and other operations can screw up the permissions of your files on your hard disk. This may cause various problems. For example applications quitting or files not opening.

OS X is already running daily, weekly and monthly crons, so it makes sense to add permission repair to one of these schedules. To do this we will make OS X execute a small Unix command diskutil repairPermissions / daily.

Download the file below to your Desktop and unzip it. Open Terminal and type in the two commands below. (Replace yourusername with your username. ;)
sudo cp /Users/yourusername/Desktop/daily.local /etc
sudo chmod +x /etc/daily.local

The first command copies the downloaded file from your Desktop to your invisible /etc folder. You will need to give your admin password after the first command. The second one gives the system access this file.

From now on your permissions will be repaired every night. If you want it to be just weekly or monthly, just rename the file.