You must wait until this user responds before you can message them again.

@ArishMell I don't remember ever seeing that. What triggers it?

@ravenhill @ninalanyon

It is not a new feature and it appears only if you try to do what it tells you not - send two messages sequentially.

I have triggered it accidentally once or twice, but it doesn't seem to happen all the time so might have a time limit on it.

Administrators, where are you? Please clarify it for us!