You can try the following options:
- Repair the file permissions from the Disk Utility application;
- Start Aperture with the Alt + Cmd buttons pressed to open the ‘Aperture Library First Aid’ dialog and run the Repair Permissions and the Repair Database.

Maybe this should help.

Patrick’s suggestions aim at the library. To me it looks like a problem with the Aperture app itself. I would try resetting the application preferences first.

1. Quit Aperture if it’s running. Navigate to the Preferences folder in your home folder and move this file to the desktop temporarily:
/Users/username/Library/Preferences/com.apple.Aperture.plist

Note: If you’re using OS X Lion, your Library folder will be hidden. You can get to it by holding down the option key while selecting the Go menu in the Finder.

2. Restart Aperture and check for the problem. If the problem is resolved, you can trash the preferences file you moved to the desktop. Aperture created a new one. You’ll need to go through the Preferences dialog in Aperture to reset any changes you made from the default. If the problem is not resolved, you can put the old preferences file back if you wish.