What most people don't know about CO2 is that it can exist as a liquid and a gas.  Sometimes, it exists as a combination of both while in your tank.  If liquid CO2 drips into your paintball marker, it can cause problems.  It will cause unpredictable spikes in velocity causing your shots to be inconsistent; some balls will lob, some fall short and some normal.  Sometimes liquid CO2 can even cause damage to the o-rings in your gun.