If participant Bob blocked being invited ("Prevent from being invited to appointments" in preferences) and Alice tries to invite him in the web GUI, she gets an error message indicating the problem.

If Alice tries to create an appointment in an CalDAV-sync'ed external application, there is no such error message, and the behavior depends on the application used:

• Thunderbird 31.7/Lightning simply refuses to create such an event
• iOS 8 creates it, but it does not get sync'ed at all (stays on the mobile device) -- this is even worse, as the user does not even have an indication something went wrong

Very likely other clients are also affected.

Proposed solution: the only reasonable way to send error messages to a broad variety of clients would probably send a mail to the inviting user.