Here are a few incompatibilities for indexing between Matlab and Octave

1)

z = zeros (5,0);
z(3) = 1;

Octave throws an error, while Matlab results in a 1x3 vector.  Interestingly, Octave gets the same result as Matlab (1x3 vector) if "z = zeros (0, 5)" (see also example below where column index of 1 seems to be special).

2)

z = zeros (0,1);
z(3) = 1;
size (z)

Octave returns a row vector ([1, 3]) while Matlab returns a column vector ([3, 1]) for this case.

3)

z = zeros (1,2,3,4);
z(5,2) = 1;
size (z)

Octave throws an error, while Matlab returns [5,2,3,4].

4)

z = zeros (1,1,3);
z(5) = 1;
size (z)

Octave throws an error, while Matlab returns [1,1,5].