When I reset the develop settings of a photo (if it actually has develop settings applied) when in full screen, the window size is reduced (thought the develop settings reset is successful). If I do it when the window is not full size, then an "illegal operation" type sound is made.
I'm using the latest version of Lightroom (2015.4) on a Mac using OS El Captian 10.11.4. I remember performing an exposure reduction but don't think it was specific to that. The first time it happened there were probably many develop settings applied.
The problem did resolve itself after restarting Lightroom though.