undecidability

Unicode's transliteration rules (UTS #35) are proven to be Turing-complete by compiling 2-tag systems, showing termination is undecidable. This result affects the ICU library used in many systems.

Research from the MIT Hardness Group proves that Super Mario levels can be undecidable, meaning no computer program can always determine if Mario can reach the castle, placing Super Mario in the hardest complexity class.