When application access is restricted, managed Remote Desktop clients may see a blue screen when the screen is locked, and the screen may not be unlockable.
Restart the client computer to gain access to the computer.
To avoid the issue, if you're using Parental Controls to restrict application access, enable the "Allow Supporting Programs" option in the Finder & System settings.
If you're using Managed Client settings in Workgroup Manager on a Mac OS X Server, enable the "Allow approved applications to launch non-approved applications" option (also known as sub-launching).