Even inside a C extension where the Python API feels like it gives you control over when you release the GIL (with functions you'd have to call explicitly to release the GIL), it turns out that:

* any operation that allocates new Python objects might trigger garbage collection

* garbage collection may run `__del__` of objects completely unrelated to the currently running C code

* `__del__` can be implemented in python, thus releasing the GIL between bytecode instructions

Thus there's a lot of (rarely exercised) potential for concurrency even in C extensions that don't explicitly release the GIL themselves. nogil will make it easier to trigger data race bugs, but many of them will already have been theoretically possible before.