Let me explain the scenario... Today a user messaged me and I tried to reply him but as I click the reply button, the page reloaded and I saw page not found.
Then I visited that user's profile and found that he has turned off private messaging. That is why I got "Page not found" message.
I think, instead of page not found message, there should display "The user has turned off private messaging." This will give clear idea to rest of the users what actually is happening. Because I am running Q2A site so I know that this is the problem but not everyone would be knowing this issue.