Permissions don't really "go bad" like an old sandwich you left in your desk drawer too long. The permissions on a file or folder contol who can see, read, or write the file or folder and they don't change by themselves. A program can set or change permissions on a file or folder. The problem is when one program or person thinks permissions should be set one way and another program or person thinks they should be another way.
Example #1: When you create a file, it might have read/write permission for you (which makes sense), but not the permissions you intend for other users. If you don't bother to check or set permissions, another user may not be able to open a file you wanted them to be able to open, or they may be able to open a file you wanted protected. If you are the only user of this Mac and always use the same login name, you don't care about this.
Example #2: Suppose Mac OS X comes with a file, set with certain permissions, but a third-party application you install changes those permissions. Mac OS X may not like this. Or, conversely, both Mac OS X and the third-party application might be happy until you install a Mac OS X update, which can reset the permissions and cause the third-party application to choke.
You can change the permissions on a single file using the Get Info window or (for brave or careful people) with the Terminal application.
You can change many permissions at once by running an installer or a utility program. I heard that the most recent version of the Disk Utility application has a Repair Privileges option and I think the Mac OS X 10.2 installer has this option too. Can others confirm this, since I've never had to use them myself?
