The little dude is part of the WordPress theme I'm using on the homepage. It's that theme's custom 404 page. I could hack it and change it but if/when the theme updates I would lose my changes (and/or have to remember to redo my changes after the update). Just more work.
When there's an error on the forum it kicks it back to the root directory,
www.bonnevilleamerica.com, where the WordPress site lives. WordPress doesn't know what to do with the forum code, all that jibberish that starts with /forums/..., so we get the 404 error page.
It's all just another reason to kick my butt in gear and upgrade the software, which will probably also eliminate WordPress (and that theme) from the equation.