If you're a coffee drinker, you have inevitably made a pot of coffee a little too strong or purchased a cup of extremely bitter coffee at some point. According to ClueDB user L2G, a pinch of salt will dial down the bitterness.
While it won't make a difference for people who already think coffee is too bitter, it will lessen the punch of a poorly made pot of coffee. Just make sure to only use a tiny sliver of a pinch, otherwise you'll taste the salt, which just ruins the cup of coffee. Alternately, you can get beat the acidity and bitterness from the start with Food Network's suggestion of adding a pinch of kocher salt in a French Press.
Cut coffee bitterness | ClueDB