Something in your teeth
Good friends don’t wait to tell friends about the bit of parsley stuck in their teeth.
It’s always kind of awkward, but in the end, they’re your friend, and you want them to always look their best. Not only that, you want them to feel comfortable telling you (on the spot) if it ever happens to you because you also want to always look your best.
And so this is how you build a culture of trust, by creating an environment that makes it routine to address the elephant in the room (or the parsley in the teeth) as soon as anybody notices.