The JAR POF exam often includes the twin to this question:
" Following a single engine failure, you use rudder to arrest the yaw and hold the nose towards the live engine, You then sideslip down the original track using ailerons to keep the wings level. What willl the slip indicator show?" ( I have added extra words to make it more inelligible than the JAR question really is)
Once again it will tell lies. This time you will be slipping but the ball will be central.
The moral of the story is if you trust your slip indicator following a single engine failure, don't do the JAR POF exam.