I found two errors with the moon charm:
http://imgur.com/a/15g03
The first picture shows my test setup.

• I was able to teleport up onto blocks that are according to the wiki illegal (picture 2). This only happens if I stand on the border between two blocks (as shown in the picture).
• If I use the charm below the grass block next to the mushroom, I end up on top of the mushroom (picture 3). This happens even if I stand in the middle of the block.

In beta4, I think this problem has been slightly improved
However, when converting the player's coordinates (double → int), the deviation of the coordinates would continue to occur.