How did the Great Hall get the Enchantment? Well, it wasn't done on purpose!

Argus Filch lost his memory in 1978 or was it 1978? Whichever it was, he had forgotten a lot about himself when it happened, so it surprised pretty much everyone when he showed up to the Final Battle and was casting spells.

Roger Davies and Dudley Dursley had been dating for five years and now a chance encounter had caused Dudley to run late to pick up his boyfriend for their five-year anniversary dinner.