One situation where I'm able to reproduce it is by doing the following:
- Click on "view year" for a class.
- Copy the URL you see in your browser. It looks something like this: https://www.lds.org/lesson/#year/nnnnnn/2013 (where nnnnnn is replaced by a real number)
- Paste the URL into a different tab. I would expect it to open to the year view of the selected class. Instead, I get an eternal spinning ball.
I've seen the spinning ball appear today just by clicking on some things without doing the copy/paste of the URL, too. The above is just a way I'm able to reproduce it.