This was me a couple of months ago!
My phone is always on silent. It vibrates, but I never have sound on for notifications or calls or anything else aside from my alarm in the morning.
So I get to the theatre, I know my phone is on silent. Sit through the first half, interval, halfway through the second half my phone starts playing a song.
It was the alarm I'd set a pissing week ago to remind me to do something that night. I thought I'd set it as a one off, but no, I'd set it for every bloody Thursday at 9.15pm.
It was absolutely mortifying, I'm still cringing now recounting it.
I do think that most of the time when someone's phone goes off, they've genuinely forgotten to set it to silent before the play or film started. Yes, some people are dickheads who think the rules down apply to them, but for most people, it's a genuine mistake.