Sometimes I read from musicians who worked with Bob Dylan, that unannounced he changes time signatures. They make it sound like intentional testing them. I have had that experience where someone I’m backing up changes time or the key or the arrangement. My sense is they forgot the time signature, accidentally started in 3 or 4 or whatever and if they realize it is incorrect they don’t want to apologize to the audience or the band. Embarrassment is so powerful. Like in World According to Garp, John Irving wrote Garp’s mother found embarrassment got more respect from people than asking nicely ever did.