Oh, this should make things a bit more interesting. Apparently, if you go through a door that’s on top of spikes, you end up becoming completely invincible when you exit the other side. Here’s GameExplain’s video on the matter, showing off a level that uses the trick:
Okay, it’s a bit complex than I made it sound. You need to be a Mario form that turns back into Super Mario when you enter the door (so you need a cape, propeller suit or fire flower for this). You need to enter the door around the time the temporary invincibility wears off after taking a hit. But either way, it’s not a difficult trick to use, and once it’s done… any difficulty in a normal level is absolutely and utterly trivialised.
Add how the glitch is likely to be required for certain user made levels (like any new trick or finding is in a game with a level editor), and you’ve got a glitch that’s going to really makes things tough for Nintendo here. Do they patch it and hope they can catch every single level online that might be using the trick (otherwise 100 Mario Challenge could well be literally impossible afterwards)? Do they leave it and hope it doesn’t get abused to hell and back by every remotely difficult level out there? You can argue either way to be honest.
What do you think about this unusual Mario Maker bug?