/effect @p 8 999999 255 negates fall damage. However if you jump as you take damage you jump higher than 1 block you jump about 60 blocks instead. Things I have tested so far and have behaved like this are ender pearls, fire, cactus, lava, poison, instant damage, tnt. How it seems to work is if you are in midair after a jump it launches you (only after a jump, if you walk off blocks it doesn't work). Fire and poison launch you even higher as you take more damage after the jump.
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TheXDragon_LTU
This is not a duplicate, because the whole report is about mining fatique, i don't know why the name was changed from "Cannot break under mining fatigue 5" to "Effects with insane amplifiers" if it does not say anything about other potion effects. This post is about other effect, where a 255 multiplier has a side-effect(launched into air when damaged). When you give your self jump boost 255, you jump normaly and don't take any fall damage, but there's an annoying side-effect that is not intended. I would suggest re-opening this entry.
migrated
yet it is resolved as a duplicate because both level amplifiers of these effects are NOT supported
Unfixable. MC-10755