A pet peeve: people who say you should learn Latin in order to understand basic grammatical theory and make it easier to learn other languages.

If you want to learn Latin for geek value, for fun, to be elite, for vaster academic purposes, that's fine. But if you want to understand basic verb tense theory in order to learn European languages, just studying any European language will do just fine! Latin does have the advantage of Romantic word origins (because I always forget if that's entymology or etymology, and the other one is very icky) in combination with Germanic/Slavic cases, but neither is that difficult to learn on its own, and if you're never going to speak a language that doesn't have cases and declension there's no point in learning one that does just to help you with your verb tenses.

The worst instance of this I've ever seen was someone asking a web community whether they should learn French or Latin. So many people advised them to study Latin, because then it would be much easier to learn other Romantic languages. Um hello, the same thing happens if you study French, plus you'll end up speaking a viable, living, European and world language!

This over-enthusiasm for Latin is akin to encouraging someone study Harmony and Counterpoint before even picking up an instrument. Or learning how to balance chemical equations before mixing baking soda and vinegar to watch them explode.