Try to type the following code in a JS buffer:

function test ()
{
    return /[/]/.test ('/')
}

This function is legal in JS and returns true. However, Emacs is badly
confused: indentation, syntax highlighting, C-M-f over the regex all don't
work correctly.

Note that while _this_ regex is silly, it is based on a real-life one that
uses a character class with a slash in it.

Recent Emacs trunk.

Paul