If your ass is not making it to the session on time at 2 pm like we agreed on multiple times in a row, I will not trust you to be on time for an event.
If you are making absurd demands of the DM and try to bully them to include your overpowered item (including homebrewing it for you and then you throw a fit when you don't like the results), I will not trust you with real power. etc etc
EDIT: I am not looking for DMing advice here, I already do sessions 0, read articles and have dmed and played a lot. I am mostly noting how small shit can tell you if you can rely on/trust somebody for more serious stuff and that TTGs have a lot of small shit coming together at once.
I wholeheartedly agree.
It also works as a first line of defense; some especially toxic
types get mad at the concept of a session zero. I'm not kidding; I speak from personal experience. The source of the rage is apparently from their suspicion that the game will be a "safe space/hugbox" and them getting mad about even the idea of people consenting to content helps screen them out that early on. 
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