"Never leave a hat on a bed" from Drugstore Cowboy.
This one always stuck with me since I first saw the movie, but I've never been the superstitious type. I've always kind of tested the boundaries of whether they have actual repercussions in real life. In college I kept all of my hats, like 5 of them, on my bedpost because I was sharing a dorm and had no space. That turned out to be the single worst year of my life and the consequences of things that went on then still effect me to this day. All of that bad hat energy gathered there causing me far beyond just 31 days of bad luck. I'm still trying to lift the curse now, almost ten years later. Whenever I see a hat on a bed, I don't assume the next month is going to be shit for me, but I won't let it stay there for long. It seems to me that supersitions are usually based around breaking some kind of tradition, and traditionally hats were never supposed to be worn in the house. So to go as far as to leave one on your bed is like crossing that line between your interior life and the outside world that trips up your karmic wiring.
Linklater probably has a handful of these. Superstitions illustrate the colloquialisms of characters and the more we relate to the film's logic/worldview, suddenly we find this logic brings truth values to our day to day. I'll post more clips as I come across 'em.