It was the back button.
When I hit back button, the whole page is refreshed, and jQuery selectors are refreshed too.
e.g. if I have this in my coffeeScript:
$body = $('body') console $body, $('body')
When I move forward in the page, they console.log the same thing, however when I hit back button, the two are not the same thing any more. The dom was somehow refreshed.