Using Bronze Sphere may result in unexpected extra experience, can be used to cheat. Sometimes causing more serious problem that the experience bar just dissapear because of the negative experience point.
It is tested that the bug is not related to the number of experience contained in the Sphere.
Here is a case about the issue. In this case, I was trying to contain xp into the Sphere:

Here is the xp data of before, as and after the bug was triggered:

As you can see, the total xp was decreased, while the level suddenly jumped to much higher than before.