The spec says that a 'return' from a thread should raise a LocalJumpError. Looking at the code for RETURN_NODE in compiler.cpp, the question of "Is this a thread" is never asked. And, I guess this exception is only for the top level block of the thread since a function called from the block could do a return. I looked briefly at the Thread create process and I didn't see anything that flagged the block passed as the top level block for the thread. The testcase causes a segmentation fault. I looked at the signal handler too and it appears as if it is "lets do this for now and address it later" code. The SEGV fault handler simply sets a flag and returns. The return will put us back where we were just at so we immediately seg fault again and then we do an exit -- with no core file. I don't see the value of catching SEGV in the first place unless the underlying Ruby code has asked us to do that. I think that would generally apply to all signals. Perry