I read this very interesting snippet in the Wikia Super Mario Bros’ page
"Interestingly, in the very first portion of World 1-1, the developers designed it so that the a newcomer almost always gets a Mushroom. In the first level, there are blocks that the player goes under. A menacing Goomba approaches the player, and instinctively the player jumps over it. By the time the player reaches the Goomba and jumps, they will hit a ? block above that would reveal a mushroom. The mushroom goes to the right, hits a pipe and comes towards the player. Since the mushroom resembles the Goomba, the player thinks to jump over it again. Doing this, however, will almost always lead the player to jump right into the Mushroom since after they jump they hit another block from above which causes them to come back to the ground and hit the mushroom. This was to teach players that Mushrooms were a positive thing in the game."