Modus ponens is a valid "If A happens, B happens in consequence. A happens. Then B also happens". A "modus ponens fail", also called "affirm the consequence" is the invalid "If A happens, B happens in consequence. B is observed. Then A happened".
Valid: if someone is hit by a train, he dies. Someone was hit by a train, then he is dead.
Invalid: if someone is hit by a train, he dies. Someone is found death, thus he must have been hit by a train.
Valid: if someone is hit by a train, he dies. Someone was hit by a train, then he is dead.
Invalid: if someone is hit by a train, he dies. Someone is found death, thus he must have been hit by a train.