Revealing mistake: Throughout the series, Finola Hughes appeared as Patty, the deceased mother of the Charmed Ones. Although Patty died in 1978, she sometimes appeared in flashbacks and time-travel episodes that took place in the mid to late 1970s, and other times she appeared as a ghost. However, over the eight years of the series, Finola Hughes aged noticeably. Patty was born in 1950, so she should always have been either a young woman in her mid 20s, or an unaging ghost having died at 28.
Revealing mistake: When Phoebe throws the potion at Cole's coat, the vial doesn't shatter, even though it sounds like it. Watch closely, and you'll see the purple, unbroken vial bounce off the coat and away out of shot without any of the potion touching the coat.
Revealing mistake: In the beginning of the episode, Yenlo and Anling are fighting with swords. After Yenlo says, "I'm going to kill him, Anling. And then I'm going to kill you," Anling stabs him with her sword. Yenlos' wound is very bloody, but there is no blood on Anling's sword at all.
Revealing mistake: When Prue talks about her relationship with Justin near the beginning of the episode, she says that she finds him to be very predictable, lacking in mystery or "savoir-faire." That doesn't make sense, though, as "savoir-faire" means "know-how." She probably meant "je ne sais quoi," that is, "I know not what."
Revealing mistake: Just after Piper and Leo are taken away, Paige orbs down to the ground and runs up to Pheobe, places her hand on her, and shows a blood-covered hand showing that Pheobe's been injured. But before it's shown, while Paige is running up to Pheobe, you can see Paige deliberately keeping her hand closed and facing away from the camera, because it's already there when she orbs down.
Revealing mistake: Prue is needed in the storm drain to rescue Piper, who is stuck in a hole bottomed off by a metal circular guard. Prue uses her power to levitate the metal circle to her level. When the camera pans out, the metal rods used to actually levitate the metal circle are visible.
Revealing mistake: In the scene where The Source uses Piper's powers together on the clay object (it blows up and then freezes), after he unfreezes it, the pieces shatter and one lands on the stand thing. As the camera moves in towards The Source, before the camera passes the clay piece it visibly vanishes (as it is CGI) and then the Source holds up a piece which is much larger and a different shape.