Ender 3 X-axis belt orientation
Sadly, the Ender-3 assembly instructions were not clear on step 7, what the right direction is (teeth on inner side of the loop):



The Belt has teeth that need to engage the teeth of the hobbed gear on the motor, just like in the preassembled Y-axis. The teeth have to go aim to the "inside" of the loop. This way, the belt will not slip on the motor side and the movements will be repeatable.