About 6-7 years ago I was messing around, replaying stages when I decided to fight king boo. I started the level but didn’t skip through the first cutscene, because I wanted to watch it. However, I decided to skip once the text appeared cuz I didn’t want to read it lol. Except, when I skipped, it left the text box up, and Luigi in the boss room, except with no wall collision and the room was dark except for a spotlight in the center. I don’t know if this would be useful in any way, just though I should mention it exists.

To do it: skip right before the textbox finishes popping up when king boo starts talking

You are exactly right in how this glitch is performed and it can actually be done in a few other locations as well, for example when freeing certain toads. If I remember correctly about the King Boo one, you can't actually progress through the level so it wouldn't help in any speedrunning applications unfortunately. It's pretty cool that you found it on accident though!