1) As others have said, every new DM I have encountered struggles a lot to grasp the CR concept.

They usually reason like this:

"Okay, the party is LV 1 so monsters CR 1 should be easy. I'll drop them 5 monsters CR 1 to begin with, then in the next dungeon I'll put two monsters CR 2 and finally the boss battle will be CR 3".

Maybe due to what videogames has taught to them, they tend to believe that a CR equal to the party level is cannon fodder.


2) Another common mistake related with the last I mentioned, is treating cannon fodder as boring. As they know they are no threat to the party, the describe the battle as boring as possible, limiting themselves to call out numbers and simple outcomes, forgetting that in movies and TV. cannon fodder is always treated as something exciting, regardless of the real threat.