I made a command block system that shoots fireballs with /summon. (I was going to reconfigure the motion towards the Enderman.) I noticed that they went in an arc until they exploded and then the physics completely stopped working.
They keep on chaining higher and higher.

The exact command was : /summon Fireball ~ ~2 ~

EDIT: with the setup I have (shown in the one screen shot) any direction really will bug like this (with a few exceptions probably). Every time the fireballs collide, one of them is sent off in a random direction, however, it stays in the plane of motion in which it was fired (if that makes sense).

Also, once these fireballs slow down completely they don't collide with me, but they do collide with other fireballs.

UPDATE: Still in 14w25a, exactly as before. I haven't tested earlier than 14w21b. (I see now that "floating ghast fireball" issues have been reported before, but my issue is different. The fireballs did not come from a ghast, however, they can still be punched back at a ghast to earn the return to sender achievement.)