Tetris Throwback is the penultimate puzzle of Test Tubes. It is known for its difficulty, as the puzzle relies on chance in some parts due to randomly moving walls and panels. The puzzle gained it's name for having a remix of Tetris Theme A as the tune playing in the background. Blue states this is the choice of music since previous people who signed up complained about "missing their video games". The puzzle itself is mainly composed of various obstacles, such as acid and lava, and reuiqres mixing test tubes in order to pass some parts. The puzzle is not very wide, but is very big in height, and the entire puzzle, when viewed from the outside, is shaped like a thin rectangle. Only five checkpoints are in the entire puzzle. Tetris themed blocks appear in the level along with several pixel arts such as Mario or a Pikachu.