After playing a lot of Tetris Friends, I started getting deja vu. Sometimes, after starting a new game and placing maybe 10-20 pieces, I would think, "Haven't I seen this exact board before?" Eventually I tested my theory through brute force: I would start a fresh game, write down the first 10 pieces, then restart. Over and over and over, until finally, I found it -- a duplicate!

Apparently, Tetris Friends only seeded their RNG once, and there were only a few hundred possible seeds (perhaps 256? I didn't check). So if you got the same initial seed, you got the exact same pieces for the entire game. Tetris Friends also happened to have a highly competitive global leaderboard, where you tried to clear 40 lines as fast as possible... and I happened to have recently learned how to use AutoHotKey. You see where this is going.

I restarted over and over until I got a good seed, then carefully played through a whole game, copying my inputs into a giant AutoHotKey script. Tetris Friends was a flash game, meaning it could only process so many inputs per second, so I had to insert a short delay between each input. Testing/debugging was a nightmare too, because I had to restart until I found the same seed again! But after a few hours, my script made it all the way through a game, and bam, I was #1 in the world.

Felt real good for about a week, until Tetris Friends purged the leaderboard. :^)