You could say that it is a bug. However, it is happened when you try to simulate very high pressure and very small volume of the system.
The simulation time step is 20 frames per second. If the velocity is too large or the volume is too small,  you will find similar cases happened.

Simulation is based on model. And most model has it own limitation. I can use some trick to prevent similar case, however, user will not know s/he has reach the limitation of the simulation. So I will keep it like that.

You might want to check out [url=http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=25.msg3292#msg3292]another version[/url] (with fixed volume option).