Well, this just (almost) happened to me. I got the “upgrade to Sonoma” notification, dismissed it (hovered over it until the X in a circle appeared then clicked on the X), and… the upgrade started. I immediately restarted my iMac, and it doesn’t seem to have continued the upgrade, so I’m fine for now, but that’s most annoying. And thanks @Shamino, if I hadn’t read your post I probably wouldn’t have thought about what it was doing.
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