The lines at

are wrong. Before getting that error, the previous error may already have been default (or may not have been set at all). In that case, a "num-defaults" is decremented. This logic really needs to check the previous state more than the current, I think.

This can be triggered using only one thread, even with np.errstate:

arr = np.arange(3)


with np.errstate(all="raise"):
    with np.errstate("all="ignore"):
        pass

# gives the correct value, but uses the lookup (using threads, the opposite will be possible
# -- although not super likely -- using the "defaults" even though the defaults are not set.
np.add(arr, arr)