The following is a real example of a situation in which an incomplete is warranted and against which each student should compare his/her situation.
A woman was in a chemistry class, had done all the lab work, had taken all the mid-term examinations, and had essentially completed all the homework. The week before the end of class she was taken to the hospital and gave birth to twins. She missed the final exam and the last week of classes. In this case, she was given an incomplete, came back, took the final exam, and passed the course.
Point: An incomplete was given to someone who had essentially completed the course, but for a legitimate reason missed one or two assignments (in this case, the final exam). The requirements for her to complete the course were clear and easily identified.
Moral: If you gave birth to twins the last week of classes, I'll grant you an incomplete.
Comment: This is an example of a sufficient, but not necessary, condition. It may be the case that there are other circumstances that warrant an incomplete, but this is a legitimate example.