Maxwell's Impressive Separator
Mentioned in The Last Continent, this spell inefficiently unmixes two things using nano-demons. Archchancellor Ridcully proposed working it on salt to solve XXXX's water crisis in The Last Continent. The idea was that with a bucket of ocean water, you'd end up with a bucket of salt and a bucket of water. Unfortunately, as Ponder Stibbons points out, it takes a great deal of magic and a fortnight to produce even a pint of drinking water from a pint-and-a-bit of seawater, making it a less-effective solution than Ridcully hoped. But that doesn't mean it was actually a bad idea...
On XXXX, when asked if Bugarup University has something like it, Archchancellor Bill Rincewind mentions that the spell is known as "Bonza Charlie's Beaut Sieve". That means that whenever Maxwell invented his not-so-Impressive Seperator, it had to have come after wizards came to Bugarup. Presumably, then, many other spells common to Unseen University have different names in Bugarup University due to parallel evolution.
The spell name is a reference to an 'equivalent' Roundworld gedankenexperiment: Maxwell's Demon.