I’ve been plagued by a nasty bug in the latest version of Lightroom on the Mac. It’s affected me several times now, and the same action triggers it, so it’s not a a random one off. On top of that I’ve read similar stories online from people with similar issues. Basically, every so often, when you open the import window, the memory usage of Lightroom goes through the roof and the application becomes frozen. If you don’t force quit the application quickly after this happens, the entire system hangs.