It can happen when not all order matches have been paid. Currently such partial orders are considered expired although this may change in the future.
For example, using you may see in Overview that Status is expired. In Order_Matches there will be at least one expired partial match, which would currently mean that the order expired.
Example
http://blockscan.com/orderinfo.aspx?q=4972552
Before a partially filled order expires, it can be seen under Exchange > Markets section for the asset called My Orders. Completion shows how much of the order has been filled.