One of the many Faculty positions held by Ponder Stibbons. Only Ponder appears to have done the research into the responsibilities this otherwise archaic post entails. Specifically, it means that in the event of accidental demise of the Archchancellor the post-holder will assume the Archchancellorship until the University community has time to select a successor. As Stibbons pointed out, since no two Wizards can ever amicably agree on anything, in the event he were to become Archchancellor it would be a job for life. The Assistant Camerlengo, who would assume the post of Ponder were he to die in temporary office with a conclave of Wizards still undecided on a successor, is the Librarian.
On Roundworld the Cardinal Camerlengo in the Vatican, usually the Treasurer, would have exactly the same function during the interregnum period in between Popes. He is also charged with organising the Conclave that gathers to elect a new pontiff, and with symbolically destroying the deceased Pope's personal badges of earthly office, such as the personal Fisherman's Ring and other intimate regalia.
As in this case the Camerlengo is the Treasurer of the organisation, then logically his Discworld counterpart should be the Bursar. Is this the reason why the title was hurriedly passed on to Ponder?