We define non-expected language as the set of words and phrases which do not match exactly the syntactic, lexical and conversational expectations of a natural language processing system. We consider three different kinds of mistakes and mismatches: - lexical mistakes: misspellt or unknown words, - syntactic mistakes: phrases which are either misconstructed or not described by the grammar, - semantic or pragmatic mistakes: sentences which are either difficultly understood or in contradiction with the system's knowledge. This processing of non-expected language will be applied to intelligent word processing and conversational access to knowledge base.