A warning sign that your faith in 0-based indexing may be faltering -- catching yourself writing comments like this :)

    # element at the top right. i.e. (1, 2) aka (0, 1) in python
    A[0, 0] * B[0, 1] + A[0, 1] * B[1, 1]

That's called the "Matlab Hangover"