If two packages depend on different versions of the same package, and the package doesn't support slotting, portage will switch between the two versions on each run of emerge -uD world. This can happen, for example, when using openobex-apps-1.0.0 and obexftp-0.19-r1 together, since they depend on different versions of openobex.
*** This bug has been marked as a duplicate of 48195 ***