Resolution: Cannot Reproduce
Minecraft 14w34d, Minecraft 1.8-pre3, Minecraft 1.8
When having having wither effect you run into a funny thing whilst taking damage from it, I found it too fun to report but it messes with functionality of something I was making so I decided to report it, here are the commands you'll need to recreate this strange bug.
Note: You may have a lot of fun playing with this before you end up fixing it, but please do of course.
/effect @p minecraft:resistance 1000000 100 true (So you don't die from wither, and fall damage, not actually necessary to recreate.)
/effect @p mine craft:wither 1000000 0 true
Now jump a lot and most of the time if your motion is heading upward you will be launched about 100 blocks in the air, (Don't worry the resistance is level 100 so you won't take fall damage.)
Have fun! But also please fix!