A common but costly mistake that many consultants make is that they make assumptions while doing the requirement gathering. The sole purpose of requirement gathering is to get all the information from the business users about their expectations and their requirements. So, a consultant should not assume anything about a requirement without getting it approved by the business.

I call it a costly mistake because any assumptions made by the consultant may directly result in failure of the project. Any development or delivery made on the basis of such an assumption may get rejected by the business users, and it may result in the delay of the entire project and wastage of the man-months  used for developing that requirement.