What's the weirdest exception rule that you've encountered?

Whether in your own language or your target language, something that makes no sense, but as a native you just accept it and move on, even if you find it weird!
And as a learner you have no choice but to learn it, but for the life of you, can't figure out why is this the way it is and how it even came to be, you know? that kind of weird

For example in Persian there is exception with "KH" and "A" that you have to put a silent "V" between them in SOME words, that have nothing in common, we learn it but we never understood where it came from and why!
What's the weird exception that you know of?