We report results of an experiment on inducing communication problems in human-human dialogue. We set up a voice-only cooperative task where we manipulated one channel by replacing (in real-time, at random points) all signal with noise. Altogether around 10% of the speaker's signal was thus removed. We found an increase in clarification requests of a form that has previously been hypothesised to be used mainly for clarifying acoustic problems. We also found a correlation between the percentage of an utterance being manipulated and the use of devices for pointing out error locations. From our findings, we derive a gold-standard policy for clarification behaviour.