In my survival world, I noticed that the sound of a horse jumping will play when you press spacebar on a horse, no matter what. That means that if you are in the air, and press spacebar, the sound will play. The same applies to if you are falling or in water.

Steps to Reproduce:
Tame a horse and put a saddle on it
Mount the horse and jump a few blocks in the air
Quickly press spacebar a few times, the sound will keep playing

Observed Results:
Jumping in midair will play a sound as if the horse is jumping, but doesn't do anything else.

Expected Results:
Jumping in midair (pressing the spacebar off the ground) should do absolutely nothing.

You can see in this screenshot, I have subtitles turned on, and even though I'm in the air, the sound for the horse jump still plays.