Jumping from a platform that is two blocks tall will sometimes cause fall damage. Likewise, walking off of a platform that is three blocks tall, and falling to the ground also sometimes causes fall damage. This damage varies from half a heart to none (just hit stun), although jumping off of the three-block-tall platform will always do a full heart of damage (this much may be intended). Sprinting does not appear to affect this.
This problem is worse online, in which jumping off a platform only one block tall will cause a half-heart of fall damage (or, again, just hit stun). Again, sprinting does not appear to affect this.