In Lightroom 5 sometimes the Navigator preview pane display gets stuck on a different photo than what I'm working on in the develop module. I haven't managed to figure out when in happens specifically, but I notice it once every few working sessions.
Lightroom CC2015.12/6.12 was released today and should address this issue. Please update your Lightroom to the latest version and let us know if you continue to see the issue. Thank you for your patience.