I found this by accident. I have the beta theme on a wireframe and was mucking around having a look at it. I decided I would change the logo and, as I was already in the Media section, I deleted the image from the site while it was still selected in the logo section of the Customizer. This crashed my site with a 500 internal server error.
I took both the beta theme and its child off the server and loaded it up again. I repeated what I did above and have crashed the site again.
It is not my normal procedure to remove an image from Media before removing it from the Customizer but I was just playing around. I imagine that someone at some point will do this and crash their site. So I thought I would just bring this to your attention.