The first time I used my MES 40 (the 2nd gen model) was to break it in.  I heated it up as required. I then added the requisite amount of wood chips.  Within a couple of minutes I heard a "whump" from inside the box and there was a great deal of white smoke billowing inside.  I opened the door (bad idea. It made the flames bigger) closed it again and shut the vent.  The fire was out in a couple of minutes.  Walked in the house and ordered an 
AMNPS with an assortment of pellets.  I never used the chip tray again.  When I gave the smoker to my son to use I made it very clear to only use the 
AMNPS in it not the chip tray.   Now my problem is he hasn't brought it back yet.   I think it may be lost and I'll have to get another one.....