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Line 154 in standard library's queue.py, in the definition of Queue.put() is: 
self.unfinished_tasks += 1
This line should be protected by acquiring the all_tasks_done lock.
Theoretically, the increment could occur somewhere during task_done()!
Additional note:
Personally, I would like the increment to occur *before* instead of *after* _put(). This is because I would like to be able to implement a _put() that does not put everything. This put should be able to undo the increment. As it is, calling task_done() from put might decrease the counter to zero inadvertently.