This glitch allow you to make fireball to stop their speed to 0
This glitch can be done by following the stepst:
1. Spawn a Ghast/Go to the Nether
2. Wait for a Ghast to shoot you
3. While it it's shooting. Press ESC
4. Go to option and change the dificulty to Peaceful
5. Then Save to Title screen
6. Enter again in the game and you can see the fireball hanging in air
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Comments

This works in the pre-release.
This glitch has been around for quite a while now. I know it happens in 1.3.x, i've seen videos about it.
It will also happen if a chunk unloads with a fireball moving in it.
I've found this on many multiplayer servers and in my singleplayer world when I went exploring in the nether
This bug is present in Release 1.4.2
Please reopen, the bug still exists.
See here: MC-2342
Still affects 1.4.2 according to ticket linked in comment above.
I have also experienced this, presumably upon coming into an unloaded chunk (it was not me who 'spawned' the fireball in the first place.
I think it should be noted that the fireball was not stationary, but moving slow, although a lot slower than it should have been.