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.

Avoid getting liquid into your paintball gun by keeping it pointed upwards when you're not shooting it.  If the CO2 is liquid in your tank and you point your marker tip down, the liquid can easily drip into the valve of the gun.  You can also install an angled ASA on your marker.  An ASA is a molded piece attached to the bottom of the handle/grip of the gun that angles your air tank downward; the angle of the tank balances your marker a bit so it tips upward some.  You can also get an 'anti-siphon' piece installed on your CO2 tank; this ensures that only air will enter your paintball gun. 

Another way of avoiding liquid in your marker is by upgrading to an expansion chamber; this is an extra chamber that will catch any liquid before it gets in the gun.  The CO2 will warm up some in the chamber and the liquid will convert back to gas.  Expansion chambers are available for most paintball guns; often disguised as a realistic magazine for tactical markers.  You will know when liquid CO2 has entered your gun when your shooting becomes erratic and visible, tiny white jets/strings exit out the end of the barrel when you shoot.  If this happens, point your marker's barrel upward and shoot about 10 times until the white jets quit coming out.  Try not to point it downward for a while so more liquid will get in.