There's an annoying Apple Watch bug where if someone attempts to override your connected Bluetooth headphones, it'll become impossible to use any headphones. This bug kinda sucks because the only solution is restarting, which uses battery.
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If you're wondering what happens if you try listening to anything, it will play the song extra slow for about three seconds, then abruptly stop, and it won't play any more audio until you reconnect, but that doesn't solve the problem because it will just do it again.