Overmixing robs all the moisture from the batter, leaving the muffins dry and stiff.
Cold ingredients are harder to mix and can leave lumps, make batter flat, uneven, and make muffins dense instead of fluffy.
Blueberries sink to the bottom of the batter when not coated with flour before adding.
Baking for extra few minutes can make the muffins dry and crumbly instead.
Extra amount of baking powder or soda can make muffins rise then collapse so always use the given amount in the recipe.