Buffy walks out of Willow's bedroom but does not close the door. In the next shot, however, the door is now closed.
When Xander is asking Buffy to the dance, the strap from his knapsack appears and disappears from his shoulder.
When Willow, Jenny, and Cordelia are closing the door of the library to shut out the vampires, the doors are shown being shut from their point of view. But when it goes to the vampires point of view, the right door shuts again.
Xander goes to Angel's place to get him to help save Buffy. But there's no way that Xander would or could know where Angel was living. Xander's never been there and it's never been mentioned.
When Buffy is revived by Xander, she gasps and coughs as she wakes up. In reality, those revived from drowning first "purge" their bodies of fluid and often won't wake up for a few hours.
As the Hellmouth opens at 40:40, the Master says "Come forth, my child" regarding the hellmouth demon. In reality it's stated to be one of the last of the Old Ones known as the Hellmouth Spawn, meaning it was much older than the Master.
The Master's skeleton in the last shot is in fact the skeleton of a woman, as can be recognized by the wide opening of its pelvis.
When Xander is doing CPR we can see his hands are high on her chest, near her throat. To properly perform CPR the hands should be lower, below the breastbone. Having his hands that high would not pump the heart and would probably break her breastbone in the process.