I submitted a bug report earlier today; doing a particular thing in this particular program (rendering a model with hair and a diffuse shader if there's a Z-buffered Klieg light present) always causes a lockup. Always. No way to break out of it without killing the process; the program just sits there, calculating shadows, allocating more and more memory until it crashes.
So after more than an hour's worth of work tonight, what do I do? Run a test render to see how it looks. In the scene with a Z-buffered Klieg. Without having saved first. Giving me the horrible choice of waiting for the program to destroy all the work I just did, or destroying it myself.