Conflicting requirements
Because requirements are often collected through meetings with separate groups of users, it isn't uncommon for them to give conflicting requirements. These conflicts must be resolved prior to implementation. For example, a salesperson says that they need to be able to offer a discount when they send a proposal. However, the sales managers said that they must approve all deals. In this example, the two requirements could be compatible, but without getting clarification, you would need to make some assumptions about it being acceptable to defer sending until it's approved. Typically, the more clarity that you have, the fewer assumptions will lead to a more successful implementation and less rework.