Unfortunately I only see this thread now. Thus I just ruined a very interesting and tense 4vs4 game for my team.
I find this "feature" very unintuitive. I was happy to pay for a 12 hours extension and was not even aware of the danger since there is no warning against it, save here in this thread, which will be lost to time soon enough, setting the trap for other players.
Furthermore, it will create longer delays: today, I just needed an extension of 2-3 hours. I managed to get online earlier than I expected, so I might have finished even earlier. In the future, if I know that a deadline will be close, I will not come online at all until I can be sure that the deadline has passed, thereby delaying the whole thing (and possibly other games) even more. Not a good choice.