When Dennis says he will not burn the duster he is pouring himself a shot of liquor, in the next shot he downs the drink but the shot after the next he is clearly still holding a full shot glass.
When Mac and Charlie are on the roof and Mac says, "You're not going to be able to eat this hot dog." Charlie replies, "I'll suck it down." Charlie Day and Rob McElhenney both break character and begin to laugh before it can be edited out.