If you respond to a message in chat, which has either previously been deleted or is deleted as you're responding, the person that you're responding to does not get a notification to state they were pinged.
user 1 posts message id 1 stating
user 2 clicks reply to message id 1 and posts message id 2 stating
yay cake, but before posting the message, message id 1 is deleted
The following happens:
Message id 2 is posted, in response to message id 1, but neither the "responding to" arrow appears (which could be expected behaviour) nor the user who posted message id 1 gets notified (which I would still expect to happen, since you're still pinging that user)