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[0], $('body')[0]

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.