Revelations - S3-E7
Corrected entry: In 'Revelations', after Mrs. Post attacks Giles, when Buffy finds him he manages to tell her that living flame must be used to destroy the Glove of Mynhgon before the paramedics take him away. Since Angel already knew this, wouldn't it have been much more useful if he had warned her that Mrs. Post was an imposter?
Correction: This mistake falls under the "why" rule. Giles took a nasty knock to the head and is currently in an emergent medical situation, so it's not surprising that he isn't thinking clearly. In this state, it is more important to tell Buffy how to destroy the Glove immediately and prevent it from falling into anyone's hands than to tell her to keep it away from one specific person.
Phoenix