In addition to the normal case described above, there are several mechanized exception provisions. A student may be excused for not attending the session, and this automatically grants him or her the extension, and excuses the non-completion of the previous session's work if appropriate. A student can be excused for not getting an extension, if for example he or she forgot to obtain one in the previous session. (This can only be done if the student was there in the session: one cannot have an extension for a session with an unexcused absence.)
A student can also be excused for not completing by the next session, for which there are two levels of excuse. An ordinary excuse can be awarded by a laboratory supervisor, but it only lasts for one more session, after which it automatically expires. A special excuse is one which does not automatically expire, (although it can be manually expired) and can only be awarded by the year manager. Typically, there are only a handful of students needing these, and it is believed to be a good idea for one person to be aware of all such special cases.
Excuses are stored and collated by student name to help identify those who have obtained more than their fair share of, for example, being let off oversleeping by lots of different laboratory supervisors!