The Spirit World (Winter Solstice, Part 1)

The black and white hei-bái spirit in this episode is aptly named because "hei" is the Chinese word for black and "bái" is white.

At the very end of this episode we can see a crescent moon. However, in the end of the next episode, which happens only a day later, the moon is full. Of course, on the real Earth such a quick change is impossible, but this is only one of many curiosities of the fictional world of Avatar, where a lot of physically and astronomically impossible things occur. In fact, such a quick change in moon phases is possible even in our reality, if the moon is close enough to the planet. For example, the Martian moon Phobos goes through the full phase cycle in about 20 hours.

Referenced in Avatar: The Last Airbender: The Avatar and the Fire Lord (2007)