We follow the College of Engineering's explanation of Cornell's policy on incompletes, as described in http://www.engineering.cornell.edu/studentServices/Advising/UgradHandbook/GradesCredits.htm:
"There are many legitimate reasons for delaying the completion
of a course beyond the time allotted. An extended illness or serious injury,
for example, might make it impossible to finish by the end of the semester.
Under these circumstances, it becomes desirable to receive a temporary
grade of incomplete, and finish up the course work at a later time.
To receive an Incomplete, a student must:
Incomplete grades are granted at the discretion of the faculty member in charge of the course. Students who think an Incomplete is appropriate should discuss it with the instructor, making sure to arrange specific conditions under which the missing work is to be completed and set a deadline for mission. Having this `contract' in writing is desirable.have an extenuating reason that prevents him/her from completing the course in the time allotted; and have passing equity in the course at the time of the request. (This is generally defined as completion of at least half the course work at a passing level.)
Evidence of an Incomplete remains permanently on the student's
transcript. When the course has been completed, a grade is entered with
an asterisk, indicating that it was not completed during the regular semester.
Students should weigh the cost of taking an Incomplete against the reasons
for doing so. Students may find it helpful to discuss the matter with their
faculty advisor or a staff member in the Engineering Advising office."