In Older and Far Away (2002), Anya states that only a vengeance demon can undo their own spell. Here, D'Hoffryn states that the only way to "restore the balance" and bring the 12 boys back from the dead is to take the life and soul of a vengeance demon, indicating that there are some spells even a vengeance demon can't take back on their own.
Although D'Hoffryn explained to Anya (and the audience) that a curse performed by a vengeance demon can only be reversed by the sacrifice of the life and soul of a vengeance demon, Anya was convinced to lift a curse earlier in Beneath You (2002) with no mention of such a sacrifice having taken place.
- D'Hoffryn explains that a sacrifice must be made to restore the lives of the 12 dead boys, not to undo the wish. When Anya lifted the curse in Beneath You, no one had died (apart from the dog) and no one was brought back, so there was no need to restore any universal balance. Halfrek also lifts a curse with no ill consequences in Season 6's Older and Far Away (2002).
The flashback where Anya and Halfrek are sitting at a banquet table among dead men with a backdrop of building on fire is captioned "St. Petersburg, 1905". Also, Anya says that Russia was ready to explode, and they only gave it a push, after which Halfrek mentions that the mob was going to loot the Winter Palace. While the St. Peterburg's Bloody Sunday did take place in 1905, it was a shooting of peaceful protesters by the army and it was over relatively quickly. The events mentioned in the dialogue are from the February Revolution of 1917.
When Buffy argues with Xander about killing Anya, she reminds him that she once killed Angel. While she is using the term more for semantic effect, the phrase "killed" is still misleading. As the Slayer, she knows that, as a vampire, Angel is technically already dead, and that vampires are "killed" either by a stake to the heart, being set on fire, or beheading. Actually, using a sword, she just sent him in Hell.