In this day and age you never know when or where a new safety concern may pop up.  Thanks to the innovation of those wonderful meth heads we now have a reason to worry about our family cookouts.

Meth cooks are getting the propane tanks from the exchanges at Wal-Mart, Kroger, etc. and emptying them of the propane.  Then, they are filling them with anhydrous ammonia (which they now have a recipe for by the way). After they are finished with them, they return them to the store.  They are then refilled with propane and sent back for you and me to buy.  
 
Anhydrous ammonia is very corrosive and weakens the structure of the tank.  It can be very dangerous when mixed with propane and hooked up to our grills, etc.  You should inspect the propane tank for any blue or greenish residue around the valve areas.  If it is present, refuse to purchase that one.'  
 
For more information please go to the National Propane Gas Association’s website at:  
http://www.npga.org/i4a/pages/index.cfm?pageid=529