Since CNG is compressed at 3,600 psi there are safety concerns a user needs to know about.

    First of all the condition of the tank must be perfect. It can't have any scratches or imperfections. If the vehicle has been in an accident it should be discarded. The expiration date must also be checked and the the tanks should be inspected. The tanks must be properly secured, shaded, and protected from the possibility of cargo touching the tank. 

Also the lines must be properly run, secured, and connected. 

For more information about CNG check out CNGCHAT.com

Here is what could happen:

http://cngchat.com/forum/showthread.php?t=2899&highlight=tank

http://johnstodderinexile.wordpress.com/2007/05/27/checking-in-on-cng-accidents-will-happen/