The interesting bit that isn't mentioned is that all the given examples are actually part of the game specification, i.e. those behaviours are there on purpose.

The interesting bit that isn't mentioned is that all the given examples are actually part of the game specification, i.e. those behaviours are there on purpose. It means that if you have an accurate list of the desired features, you could probably also achieve 100% coverage. It is also possible that testing some features would not increase the code coverage. "Hard to test" in the submission title didn't mean what I thought: I though it meant it was hard to write tests for Tetris, not that it was hard to recover a complete specification of the game while playing it.