Practice-Theory dichotomy: The belief that theoretical models do not or cannot properly reflect reality, and so it is natural that when put into practice, the results will be unexpected.
This is a false dichotomy. It attempts to divide the outcome of a theory from the outcome of a real experiment. The common usage is someone saying "It's good in theory, but bad in practice". The belief is that theory and practice are so divided, that it's possible to be good in theory while still performing poorly when practiced.
When the dichotomy is rejected, it is understand that if a theory predicts a result, it should be the result when put into practice. That's part of what proves the soundness of a theory. If the theory predicts some other result, the theory is flawed and should be either amended or rejected. Theory should correspond to practice, or it's a flawed theory.