While jumping by holding the spacebar, every 2 jumps the player will glitch for a moment with feet under the block he was jumping on (only animation glitch I think)
EDIT: Only with Jump Boost effect
EDIT 2: Tested with Jump Boost II
Reproducing:
1) Drink a Jump Boost 2 potion
2) Hold spacebar
Cannot reproduce. Please describe the issue in detail and provide steps to reproduce it.