The original bug report failed to explain how to trigger the bug. I think I know of a simple way to trigger the bug, but have not yet created the simple test-case. I believe that the bug can be triggered like so:
Create some medium-sized "random" (arbitrary) snippet of guile code. Send it to the guile repl server. Do it again and again, as fast as possible. The bug will trigger.
By "arbitary/random" I mean some code that is simple but non-repeating: say, concatenate some random strings, count the number of letters in them, take the square root -- just do assorted, non-repeating computations that force the REPL to evaluate some new blob of scheme code every time.
Now, once out of every 5 or 50 times, introduce a bug in the scheme code, causing the REPL to throw an exception. Using ut8 strings might also be a required ingredient.
I believe that this will trigger the bug. I believe that the bug is some unprotected section in the guile interpreter, and a race condition clobbers the guile stack(s) and it all goes downhill from here.