Corrected entry: In 'Selfless' Anya wants to reverse her wish that killed 12 people and is unable to do it, but during 'The Wish' we were told that all you have to do is destroy her power source and all her wishes will be reversed (Anya has to have a power centre - we see in the episode 'Older And Far Away' that ALL vengeance demons have one). Even though the gang don't know this any more (they wouldn't have any memory of researching it), surely Anyanka herself would know her own weakness?
Corrected entry: In the season 3 episode "Doppelgangland", which originally aired in 1999, Anya told a barman that she was 1120. This would mean she was born in 879 AD. But in the season 7 episode "Selfless", a flashback to 880 AD showed her as a young woman (about 20). She should've only been one year old.