The Air-Breathing Mermaid Problem

Every now and then when playing RPGs you stumble upon a mechanic that solves a problem you didn't know you had. This situation is called the Air-Breathing Mermaid Problem, based on a meme that goes something like this:

A game has mermaids, they are introduced in the core book. The entire book makes you believe mermaids can breathe both air and water - there is nothing to contradict this assumption. However, when a supplement comes, you suddenly discover that you have a new power that lets mermaids breathe air. You were just given a solution to a problem you didn't know you had.

The same problem can manifest itself in different ways. You could have a poorly checked book that only lists offhand penalties to using a weapon in an ambidextrous merit that removes said penalties, or come from a specific character focus that brings a mechanic to a focus that would otherwise have been glossed over by the GM.

In our recent game of Fellowship, we had a character playing the Angel playbook. One of the core powers of that playbook, is that it can speak with the language of all things, and with some equipment, it can also understand any spoken language. Since our game is about exploring space and talking with aliens, was it not for this move, it would be a default that we can communicate with the aliens through some sort of babel fish or what have you, but with the powers that playbook introduces, either only that player can talk to most aliens, they translate for everyone and we mostly ignore that rule, or we ignore that rule and everyone can speak freely.

While this example is pretty specific, it could be similarly applied to any system that features a way to learn languages - if one player invests heavily into being a polyglot, it can either punish everyone else for not speaking the language, or punish the linguist for wasting so many points on languages.

In most other situations, it's rather uncommon to come across the Air-Breathing Mermaid Problem, but it could be a good design decision to try avoiding small mechanics and extra powers that solve very specific problems. Sure, being ambidextrous can be fun, but are offhand penalties important enough to warrant their own exceptions to the rule? Is it important that a mermaid can't breathe air? If the answer is no, maybe it's best not to solve problems nobody is having...