The wiki page on the transaction script says:
The party who originally sent the Bitcoins now being spent, dictates the script operations that will occur last in order to release them for use in another transaction
How does a recipient know they can spend the bitcoins they received? Would it be possible for a malicious user to create a script that the recipient could not use to spend the bitcoins, without them being aware of it? (Possibly due to a client UI limitation?)
The situation I'm describing would probably be unusual, as it would involve the sender deliberately sacrificing (and destroying in the process) bitcoins purely to spite the another party. Even still, is it possible, and what measures (could) protect against it?