You go to buy a table but it looks it has water damage from drops of water hitting it. This makes it a pretty mediocre table. The water drops haven't really damaged it that much, it still stands and you can use it, but it's kinda ugly, so it's just mediocre.

You weren't expecting to see such a mediocre table so you go up to the guy who's selling it and say, "Hey, I have a bone (ぼん) to pick with you!" But he doesn't think there is a bone to pick at all, he doesn't think the table is mediocre like you do.