If a player try's to climb up a ladder or vine they will move slightly forward or backwards a couple of pixels depending what direction they are moving.
Steps to Reproduce:
Get some vines or ladders and place them on the side of a wall.
Attempt to climb the ladder or vines.
Observed Result:
The player moves slightly as they begin to climb the bottom ladder/vine.
Expected Result:
The player should be able to climb up the ladder/vine without being moved.
This behavior is specifically mentioned in the Preview changelog, which is recommended reading for anyone playing Preview/Beta versions.
In the "General" section it says:
"The first person camera will now offset backwards slightly when facing and climbing blocks such as Ladders and Vines"