# Set of steps that cannot be implemented on a Turing machine

Is there an example of a "set of steps" that we thought was an algorithm, but later was shown not to be an algorithm because it could not be implemented on a Turing machine?

The appearance of quantum algorithms, in particular Shor's factorization algorithm, has called into question the efficient Church–Turing thesis (sometimes known as the Cook–Karp thesis), which states that $$\mathsf{P}$$ captures the class of efficiently solvable problems; but quantum computers are known to compute the same class of languages as classical computers.