Steps to reproduce:

1. Access PlatformUI;

2. Go to "Admin Panel -> Content types";

3. Click on content type group "Content";

4. At the bottom of the content types list, click on "Create a content type";

5. Specify the following specs for the content type:
Name: Rich Text
Identifier: richtext

• Fields:
  • field 1:
    Type: Text Line
    Name: Name
    Identifier: name
  • field 2:
    Type: Rich Text
    Name: Rich Text
    Identifier: richtext

6. Click "OK" at the bottom;

7. Go to "Content -> Content structure -> Content Tree";

8. Click on "Create" on the rightside menu;

9. Choose "Richtext" content type you just created, and name it "Test embed";

10. On Rich text field, click on "Add content", and click "h1 Heading" and write something in Heading 1 style;

11. Open Firebug or similar tool, and keep Console open to check for javascript errors;

12. Click "Publish".
You'll get an error:

Uncaught IndexSizeError: Failed to execute 'extend' on 'Selection': 8 is larger than the given node's length.    alloy-editor-all.js:408

where "8" is a size of how many characters you wrote on the rich text field + 1.