It seems the RSS widget in sidebar widgets interprets Atom feeds a little carelessly. For instance, when the feed is mixed from various feeds and has source/title inside each entry element, the widget reads not only the title of each entry, but also the title in the source sub element inside the entry and rolls them together like this:
Title of EntryThe Source
I'm getting this result in WordPress Multiuser but I'm pretty sure it's a more general issue in the widget. Can anyone confirm this? It's easy to test for example by trying out any mixed Atom feed created by Feedblendr.