Put the summary of the bug you're having here
When I went to creative mode to survival with /gamemode 0 and I pressed the spacebar sometimes to jump up I could still fly and when I stopped flying mid-air (double space) I fell down and got damage!

THIS IS IN SSP AND SMP!

Stopped occuring after 1.4.4

What I expected to happen was...:
I thought I would keep jumping up my mountain

What actually happened was...:
I started flying and when I stopped with double-tapping spacebar I fell down and got damage

Steps to Reproduce:
1. Go to creative from survival with /gamemode 1.
2. Fly a bit.
3. Go to survival from creative with /gamemode 0.
4. Try to fly with double spacebar.
5. Go like 10 blocks high.
6. Stop flying.
7. You get damage.