I leave for an hour and you guys fill the page. And you can't even see what I posted on the Bug Forums?
I'll see if I remember how to calculate chances for getting all four. Brb.
Edit 1: So I did the calculations, and, well, .00000381469 is like, nowhere near right.
Edit 2: My second calculations is giving me 1/4096, ~ .00024
Edit 3: And here is why none of us got it. Repetitive Combinations' a pain in the arse.
http://i.imgur.com/ctvZMT9.png That's the chance of getting 18/24 head flips.
Now, we can go ahead and try to figure out the chance, but I feel it is better to say that I have about a 1% chance in order to get one of these things.
Let's put this practically: There's 5 in one game (1 in Act 1, 2 in Act 2, 0 in Act 3, and 2 in Act 4 (the other one actually leads to Act 1)). The chance of getting it is 5/125, which is actually 1/25. So you still have a 4% chance to do it. You'll have to play about 25 games and visit EVERY SINGLE ONE in order to get this once. Maybe. Maybe you'll never get it, or maybe you will get it on your first try.