Yep, you're right, Tony Nelson. We overlooked this case but we can zero is_tripped after the test for threading as you've already said. The patch was updated and it also includes the code comment Tim Peters suggested. Please, I don't know if the wording is right so feel free to comment on it. I still plan to write a test case for the problem being solved (as soon as I understand how test_signals.py work :-).

File Added: signals-v1.patch