Here's an interesting bug, found by Phil. If you have a siteMapNode with a space as part of the QueryString, the node won't get rendered in the SiteMapPath. It's not actually a bug in the SiteMapPath, but in the provider and only affects QueryString arguments, not the page name. It also only affects the node if it's a leaf node, so if it has children you're fine.
Listening to Come Clean by Splendid from the album Have You Got a Name for It