In Apple Remote Desktop 3.3, Apple changed the behavior of key commands during screen sharing. In this knowledge-base article, one of the listed “improvements” is this:

When controlling a remote client, function keys and key combinations for actions such as Force Quit, Log Out, and the Application Switcher are now all sent to the remote computer.

While this feature has its benefits in some scenarios, some people may prefer the old functionality. Here’s how to make Remote Desktop not send the special function keys to the remote computer. Quit Remote Desktop, then open Terminal and enter this command:

defaults write com.apple.RemoteDesktop DoNotSendSystemKeys -bool YES

If you decide you want to have the special function keys sent to the remote machine, just quit Remote Desktop and repeat the above command, but change YES to NO.