Invisible walls make more appearances than the player might think in some, if not most ROM Hacks. In the original game, these were just used for the outskirts of each level, bordering where Mario could and couldn't go. (Although it was pretty clear nonetheless, with steep slopes and/or cliffs marking your certain death or clumsy and difficult path only to lead you back down to the stage.) If invisible walls weren't meant as a troll in a ROM Hack, it was most likely put there by accident. Or, the developer(s) thought it would be completely justifiable to put a wall where a simple shortcut could have been made. Linearity isn't something that's expected in most hacks, so it would be a quick thing to adjust to.

SimpleFlips has had his fair share of encounters with these problem-causing barriers, usually resulting in his confusion as to why his midair/grounded velocity has just been halted to a complete stop, along with a viewer or several to scratch their heads in wonder, asking: "now why would a wall be where you could easily maneuver to safety without it?". On occasion, some walls can bump you into your further demise, causing the player to sink helplessly into quicksand or lava. Or even worse, if it's really just your luck to fall into the iron-grip of a Gay Baby Jail, then these invisible walls have really just proven themselves to either hinder your playing experience or keep you from doing that to yourself on purpose.

However, because these walls just act as simple barriers, one could miraculously BLJ to clip through it. That is, if the wall isn't somehow reinforced to stop the player in his "yahooing" tracks. There's also the issue of seeing if a slope is in the nearest vicinity, let alone one that's steep enough that'll give Mario enough speed to go through. In short, invisible walls are something that were originally meant to be a practically nonexistent part of a game, and were then turned into source material for ROM Hackers, be it to cause annoyance or implement them for their intended and more efficient purpose.

It's well past possible that the invisible walls that randomly appear in hacks are just the collision building incorrectly, which means the level importer needs more work. That is, if SKELUX doesn't add anti-piracy to it...
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