A bugbear: iOS' default mail client having the "Mark as Read" and "Move to Junk" actions on an all-selection right next to each other (with no undo option) is maddening.
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I know that Instagram has this silly feature of "shake to show a 'report a bug' prompt" - I know it because it occasionally turns on for no apparent reason.
It's even less discoverable than any undocumented gesture out there. Percussive maintenance is a joke, and the only people who actually shake something in anger are fictional characters in movies and animation, because that's done for comedic effect. And maybe people with serious anger management issues. Neither connotation make this gesture a sensible choice.
It’s not discoverable, but it is in the user guide.
If you find yourself accidentally activating it, you can turn it off in accessibility settings.
If you would rather have a button for it, you can turn on assistive touch.