I have a very jQuery heavy app which I am including in an iFrame on a site I am working on. Developing in Firefox and Chrome, it was only at the last minute when I came to test the site in Internet Explorer. Much to my dismay I was getting “$ is undefined”. There were no issues when running the site directly in IE (not through the iFrame).
I tried numerous fixes recommended by Stack Overflow including delaying the loading of the iFrame until the main content had loaded, but in the end, it was simply fixed by hosting jQuery from my own server rather than pulling it in from the CDN.