Turing's proof that no algorithm can determine in general whether an arbitrary program halts. The foundational theorem of computability.
From Wikipedia
In computability theory, the halting problem is the decision problem of determining, from a description of an arbitrary computer program and an input, whether the program will eventually halt or continue to run forever. Alan Turing proved in 1937 that the halting problem is undecidable, meaning that no general algorithm exists that can correctly solve the problem for all possible program–input pairs. The problem comes up often in discussions of computability since it demonstrates that some functions are mathematically definable but not computable.