When editing files in LrC v14.5 or 14.5.1 sometimes an ! mark will appear on the thumbnail and an error message "Lightroom has encountered problems reading this file" and a message that you can no longer edit the file. You can find a report on the Adobe site at this link.
I have encountered this problem a few times and have found that it is not possible to backup in the edit command stream and recover. I did recover by going to Windows Explorer and duplicate the RAW file. Then I import the duplicated file and sync the edits from the old file to the new file in a sequence of steps to see which edit is causing the problem. I find that it is always mask editing that causes the problem for the few cases I have encountered. The error also occurs when editing virtual copies, in which case the virtual copy cannot be recovered, another virtual copy can be created and sync'ed with the first virtual copy, except for the masks, to recover. I found that for a few cases it was possible to recover one mask without getting the error, but not the second mask.
Regardless, there is a bug and the way to avoid it is to go back to version 14.4.
