RHX5 - The default topic displays in the ToC and content panes

A colleague had a curious problem recently. When you open the Start Page of her RH project, you get the traditional tri-pane. The ToC is on the left and the content (Default Topic) on the right. Clicking the first book in her TOC displays the Default Topic in the left (ToC) pane, so you view the Default Topic in the left and right panes.

Any idea what the problem is?

More details:

• Created and compiled in RH X5.
• There's a Start Page (e.g., abc.htm) and a Default Topic (xyz.htm). The Default Topic, when generating the project, is xyz.htm. The TOC starts with a book (let's say the title is AAA) with a link to xyz.htm. There are several other books (Getting Started, etc.) under this first AAA book. I realize it's unusual to have a book first, and not a page, but that's what she had.
• Ran the help on IE 7.0 on a WinXP Pro 32-bit system.
• 508 Compliant Output was not checked when generating Webhelp.
• There were no Start-specific scripts in her non-Start topics.
• The colleague did not edit her output files.
• The project was created, from scratch, in RHX5. She did not recreate it using the output files.