When debugging with gud and gdb in many-windows mode, some source files are opened in the gud interaction window instead of the source window.
This appears to happen to files, and only those files, which were open in Emacs before gud was started.
I've replicated the behavior without my environment using
This is currently in version 24.5.1 but I also experienced the behavior in 24.4.
My bug report is here: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=22374