1. Cannot Be Seen From the Moon
Not even from nearby space.
This misconception has been popularized by Robert Ripley in his book "Believe It Or Not!".
It was meant as an exaggeration of the size and length of the wall but with no evidence.
Ever since people have picked up on this and soon became the most famous myth about the wall.
This can be easily verified using any satellite imaging tool like Google Earth for example.
2. Not a Single Wall
The great wall of China is not a single continuous homogenous wall.
It is made of tens of independent walls built at different times by different people.
The walls have gaps between them and occasionally get connected by towers and fortresses.
3. It took more than 500 years to build
It is difficult to tell exactly when the construction started because the Great Wall of China was not commissioned as a single project with a clear vision.
It all started around 200BC when Qin Shi Huang gave instructions to connect already existing defense towers.
Work started slowly. It was not until the thirteen's century that the majority of the wall seen today has been built.
4. It failed its purpose
The reason the wall was built was to protect against invaders.
While it succeeded in some occasions, it failed in others as in 1449 when the Mongols managed to cross the wall and defeat the Mings.
5. Only 2/3 of the wall remains
The remaining third has vanished throughout the years.
Where did it go? No one knows but it is speculated that people through history took the stones and bricks from the wall to build their houses while the rest has been hidden by changes in the landscape naturally.