Everything that makes Super Mario Bros. 3 truly special is exemplified, for me, by a single image: A chain chomp breaking free of his bonds and bouncing off the screen.
You'd be forgiven if you didn't know that chain chomps - the toothy, black, ball-on-a-chain cum guard dogs that first appear in level 2-5 - could actually break free from their eponymous chains. A chomp has to lunge a full 47 times for its silver chain to start flashing a distressing red, and three more lunges for the chomp to finally break free and bounce towards Mario. The entire process takes about 175 ticks of the in-game timer, depending on how much Mario goads the chomp. That's easily seven times as long as even the slowest of players usually takes to jump past those snapping teeth and on to the next challenge, without a second thought for the poor, imprisoned enemy they're leaving behind.
On first glance, this hidden extra seems like a relatively meaningless addition to the game, the kind of pointless Easter egg a bored programmer might have thrown together during a coffee break without anybody else noticing. But taken in the context of the game as a whole, that chain chomp's potential for freedom is emblematic of the way Super Mario Bros. 3 subverts players' expectations to make a game that feels truly magical.
It starts in the very first level, with an item block that's stubbornly attached to the ground. Immediately, the player has to throw out the notion ingrained by the original Super Mario Bros. that all such blocks must be attacked with a jump from below. Then there's that familiar diagonal line of coins over the game's first major gap, inviting the player up into the sky, practically begging them to figure out how to fly higher than they've ever been able to go, unassisted, in previous Mario games.
The game hints at this potential with another seemingly meaningless touch - the way Mario sticks his arms out, like a child pretending to be an airplane, when he's achieved liftoff speed. Even without a raccoon tail equipped, Mario's jumping animation is slightly different when he leaves the ground at this speed, his feet coming to a graceful V behind him, his body rounded to achieve maximum aerodynamics.