When you mount a moving small fireball, the game thinks you are riding it but nothing will happen. If you try to dismount it you will be teleported to the fireball before dismounting it, like if you were mounting it before.

How to reproduce:
1- Put a dispenser with fire charges and a lever behind.
2- Put a repeater on the side of the dispenser, pointing towards a command block with the command /ride @r mount @e[type=small_fireball,limit=1]
3- Pull the lever and see how the fireball goes without you, however, you will see in chat as if you were mounting it.
4- Run /ride @s dismount. You will be teleported to the fireball as if you were mounting it before.