I tried to use app pinnigg once. Foolishly, I believed MIUI could handle me pressing the pin button.

Turns out it wanted a key combo to unpin the app, but I had turned on gesture navigation so the buttons never appeared.

A few quick reboots let me change the setting back before the phone realised the app should be pinned and let me fix the problem, but it shows how these little-tested features can get overlooked!