When I'm browsing phpBB.com as guest and want to send reply to topic I click on "Post Reply" button and it leads me to login page. After login I'm not redirected back to write reply, but I see message "The forum you selected does not exist.", so I have to find the topic I want reply again.
It seems that I can click on Back button on my browser and it get me on page where I can write reply.
Before I was redirected back to writing reply.
EDIT: The very same thing happen when guest use quote.