Mounting a horse also uses a throwable item in your hand at the same time.

[alpha 1.1.0.3 332621]

Summary:
If you are holding a throwable item in your hand (Like a snowball, splash potion or ender pearl) and right click (or click 'ride' in PE) on a horse, you will also throw and consume the item as you mount the horse. With an ender pearl, you will actually teleport off of the horse too.

Impact: High priority; player may kill themselves while mounting a horse (Main mechanic in this map)

Repro Steps:
1. Load any fresh world in W10 or PE
2. Spawn a horse
3. Place a throwable item in your hand (Snowball, splash potion, ender pearl, etc)
4. Mount the horse

Observed Results:
You mount the horse
You also throw the item

Expected Results:
You mount the horse
Your item is not thrown/consumed.

Screenshots/Videos attached: No