One example is one of the first bugs I ever fixed as a working developer. A click event was broken on Firefox only because of a nested anchor and button element

One example is one of the first bugs I ever fixed as a working developer. A click event was broken on Firefox only because of a nested anchor and button element. As interactive elements, they shouldn't be nested according to the HTML spec, but it's common for developers to reuse button styles on a link by wrapping it with the button element. The behavior is undefined, which resulted in it working okay on Chrome but not Firefox.