These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Super Mario Bros Crossover
Demonic Spiders: In addition to Lakitus and Hammer Bros., the Buzzy Beetles acquire this status due to their invulnerability to everything except the Goomba Stomp.
Later versions make them vulnerable to certain special weapons, mainly ones with limited ammunition to make up for the fact that not all characters can stomp in them.
Taken Up to Eleven in the most recent update with two cheat options: 'All Hammer Bros' and 'Evil Hammer Bros'. The former turns every single enemy in the game into Hammer Bros except Bowser, Lakitu (who throws Hammer Bros) and Bullet Bill Launchers (which fire Hammer Bros). The latter makes Hammer Bros chase you relentlessly throughout the entire level - even though you're sure you left them in the dust at the beginning of the level, they just keep on coming. Combine both for every platform gamer's nightmare.
Earworm: The "dubstep" remix of the Super Mario Bros theme when you select a character - the high score music in Vs. Super Mario Bros, an arcade version (In 2.0, it only applies to the 8-bit mode). The other character select themes are pretty catchy too.
Game Breaker: Bill has a decent height jump that is easy to compensate for the slight forward movement he has and a weapon that can take down most enemies in a decent number of shots and gets even better when given power ups.
By "better" we mean "it's a rapid-fire spreadgun that destroys bricks and fires in eight directions". Bill didn't get upgrades because nothing could match that gun he carts around.
The developers are aware of this and to make up for the massive firepower he can pack, he can't take hits or get mushroom power ups, a nod towards Contra.
And it looks like Sophia the 3rd will be even worse. It comes with all of its abilities (That is to say, Hovering, Wall/Ceiling Climbing, and free underwater movement), plus homing missiles. It was nerfed in 2.0 by having the climbing and hovering abilities be seperate powerups instead of innate abilities, just like in the actual Blaster Master games.
As of 2.0, Bass is arguably SMBC's new game breaker. Let's recap: standalone dashing (which can be canceled into a bigger and higher jump), double jumping, autofire in seven directions with power equal to Bill's machine gun (and faster). All of these abilities available just by choosing him. Just grab the metal blade, and the game well could be showing you the "your quest is over" screen.