Minecraft 14w02b, Minecraft 14w02c
Windows 7 Ultimate Service Pack 1
When i was trying to give myself a negative levels. Then the bar just'd disappear until another time you give another negative levels. This happen to same versions of 1.8 snapshots.
1. If you have cheating on inside your world, you can proceed. If not, open lan mode with "Cheating on"... Then proceed to step 2
2. Type on chat "/xp 1000000000L <your player>" then it gives a negative level without the bar showing off...
To fix this glitch off, just do "/xp 1000000000L <your player>" again until the Level bar goes back off to 0.
If this glitch was already reported, then please take care of this problem.