If you have any of the following, then you can defer an exam
The University defines an examination clash as:
two examinations at the same time including any overlap.
less than two hours between exams where they are held on the same campus.
insufficient travelling time (less than 3 hours) between examination venues at different campus or off campus sites.
where you have more than three examinations scheduled on two consecutive days. The University does not regard Friday or Saturday and the following Monday as consecutive days.
An evening examination that finishes after 8.30pm followed by a morning examination that starts before 9.30am on the next consecutive day.
Anyway, as for my missing exam, the examination people lost the subject and forgot to put it in the timetable