After upgrading to 2015.8 (perpetual) I found that when images in the Develop module are marked with a "X" as rejected, and then using CMD-Delete the program freezes, which then requires a restart to solve the problem.
System: MacPro 2009 32GB Ram OS 10.10.5
Please read here: Lightroom: Image stuck in Develop mode | Photoshop Family Customer Community
For the time being, until this is fixed, move to Library before rejecting an image with 'X'.
Retrieving data ...