The substitute walked in, said "just continue whatever you were doing," and the entire class instantly decoded this as unofficial permission for complete and total chaos. No further instructions were needed after that single sentence.
What followed was roughly forty minutes of barely controlled noise. Conversations happening simultaneously in every corner, someone's phone playing music quietly, general anarchy dressed up as "continuing whatever."
She never corrected course after that first statement, just sort of let it run. We consider this, genuinely, one of the better substitute periods of the entire year. Low expectations, perfectly met.