What I expected to happen was...:

I thougt that i could ride the horse, because it worked before.

What actually happened was...:

I just opened the horse-inventory and/or the horse froze.

Steps to Reproduce:

1.Tame a horse. Equip the horse with a saddle and diamond armour (may work without)
2. Sit up on the horse and ride it. Jump off after a couple of seconds.
3. Repeat step nr 2. 5-10 times. Change gamemode between survival and creative 2-6 times ( both when you ride the horse and not).
4. Change gamemode to creative
5. Try to sit up on the horse. If it works as it did for me, do you only open the horse-inventory when you leftclick on the horse, you do not ride it.
6. Save and quit to title.
7. Try to sit up on the horse. If the inventory opens, do step 6. If you sit up on the horse it meaby can't move, but jump.