Selecting a theme and pressing the back button and selecting a new theme causes an HttpException. (You may have to perform this multiple times)
Steps to recreate:
- Login as an admin user.
- Go to Administration ==> CMS Setup
- Select a new theme at the bottom of the page.
- Press the back button in the browser.
- From here select a new theme. (If page does not crash select go back to step 3)