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