This error could be reproduced when multiple users are trying to edit the same page simultaneously:
1. Log in Sitefinity administration as an administrator.
2. Open a page and start editing it.
3. Using another machine (or a different browser on the same machine), log in the CMS as another user with administrative rights.
4. With this user open the same page. There will be a message that says: "User1 is editing this page. Save changes Cancel changes"
5. Click "Cancel changes" (while still editing the page as the first user).
6. Now, when the first user clicks "Edit" on any control, or "I'm done" after editing one, the described exception will be thrown.
For now we have no appropriate resolution for this problem.
the Telerik team