Numpy masked arrays do not fill correctly when constructed. In the example below, the values filled do not correspond to the mask (notice the big chunk of values in the middle that get filled when they should not:

The same happens with other masked array constructors, such as masked_equal. Strangely, if I use nan as the mask value, it works as expected.

Running NumPy 1.11.0 on Python 3.5.1 (OS X 10.11.4)