I've seen the glitch done in many speedruns (especially Perfect Restaurant), where you start the game, buy the barrel, and while you're opening the catalog you place the barrel, then close the catalog, remove the barrel, then start the night. After the night's done, you get money ranging in the thousands. I can't recreate this glitch on my end. Are there any requirements for it?

There's a chance you're placing the barrel down to quickly after clicking on the catalog. When doing the glitch you wanna click the barrel when the entire thing has already faded out. If you'll go into it thinking about it that way, you should be able to practice it and find a timing window for it pretty easily. Removing it just requires you to be pretty fast and precise. You just need to make sure you don't end up gaining any faz rating from the barrel. If you have any more questions feel free to post another message in this thread.

Thank you for responding! In my attempts since posting this I've found that just redoing it until I have -1 ENT works best. I'm still working on my timing to get it down to one try.

How does this glitch works?? (like, in the code)

Octonine, to answer your question, the game has two functions determining the way the Pizzeria Stats are updated. These are not verbatim how they look in the code, but use a very simple Java code way to explain it:

public int AddStats(int ent_value, int rev_value, int risk_value, int health_value, int atmo_value, Item item)
{
   this.risk_value += item.getRisk();
   this.ent_value += item.getEnt();
   this.rev_value += item.getRev();
   this.health_value += item.getHealth();
   this.atmo_value += item.getAtmosphere();
}

public int SubtractStats(int ent_value, int rev_value, int risk_value, int health_value, int atmo_value, Item item)
{
   this.risk_value -= item.getRisk();
   this.ent_value -= item.getEnt();
   this.rev_value -= item.getRev();
   this.health_value -= item.getHealth();
   this.atmo_value -= item.getAtmosphere();
}

when switching to Catalogue Mode while placing down or removing an item, you basically force the break keyword into these methods, but a simple (not actual name) Boolean called is_placed is set to True or False respectively, so the game knows the Item is there or not and so you can't stack every item on one spot lol. With is_placed = True or False, that allows people to Remove/Add the nonexistent Stats from the Pizzeria.