When I run some byte-compiled code which creates some threads, and then,
while a thread is blocked, interactively evaluate the function which
was used to create that thread, Emacs has a fatal error or segmentation
fault when the thread becomes unblocked.

To reproduce:

  Build Emacs from master with this patch, in which I've pasted some
  excerpts from my current project onto the end of lisp/thread.el.  It's
  going to be like ERT but designed to run benchmarks instead of tests,
  but right now all it does is to create a buffer and three threads, set
  the threads up to communicate with each other, log their progress to
  *Messages*, and update the buffer when they finish: