We present a novel technique that automatically alerts authors of requirements to the presence of potentially dangerous ambiguities. We first establish the notion of nocuous ambiguities, which are those that are likely to lead to misunderstandings. We test our approach on coordination ambiguities, which occur when words such as and and or are used. Our starting point is a dataset of ambiguous phrases from a requirements corpus and associated human judgements about their interpretation. We then use heuristics, based largely on word distribution information, to automatically replicate these judgements. The heuristics eliminate ambiguities which people interpret easily, leaving the nocuous ones to be analysed and rewritten by hand. We report on a series of experiments that evaluate our heuristics? performance against the human judgements. Many of our heuristics achieve high precision, and recall is greatly increased when they are used in combination.