If you are running and get into a boat, some of your momentum is carried over to the boat, which makes sense. But it also happens in places that don't make sense. A good example of this is if you jump and then get on a horse immediately after. It causes the horse to jump even if you didn't jump while you were on the horse.

Steps to reproduce:

1. Get Jump Boost II (so it is more obvious)
2. Spawn in a horse. Tame the horse and give it a saddle so you can control it
3. Jump near the horse. Release your space bar immediately so you don't jump while on the horse
4. Get on the horse immediately
5. Repeat these steps until it happens. It can take a bit to get it to work.

Observed result

--> The horse will jump slightly. It is not a true horse jump though because the horse jump sound doesn't play.

Expected result:

--> Your upward momentum is cancelled out and the horse doesn't move when you get on it