I just saw something that I've never seen anything like in LR 5 or any previous version of LR. I was happily in the midst of going through images and had created a 2x2 image (i.e., I selected four images and created a 2x2 JPG in Print module). So far, so good. I went back into Develop module, and all looked fine, until I selected a new image in the filmstrip. The main image view went slate grey, and stayed that way even when I switched to Library module. I closed LR and reopened and the problem went away, though I noticed something that I've not seen before (but this I might just have overlooked); I use a color label to hilight images to be composited like I did for the 2x2 output. Those images were not colored in the filmstrip when I opened LR, but the colors came in maybe a second or two later. This is probably just a performance issue as the interface is being displayed.
I know that this probably has no "solution" but thought I'd post it because it is so different from anything I've seen in LR. I am running version 5 on a Win7 64-bit system.
Further strange behavior - other aspects of the filmstrip aren't being displayed. Stars don't show up until I hover over a particular image in the filmstrip and then, after a second or so delay, I see the star ranking. I couldn't get the badges to show up until I clicked on an image in the filmstrip and now suddenly all the badges are displayed. Very strange!