This happens with Octave 4.2.1 and image-2.6.1.

Consider following example code:

im = magic(15);
out = imrotate(im, 50,'crop');

This throws an error in Octave, because 'crop' is no valid interpolation method. But it works fine in Matlab. See the second code line here, for example: http://de.mathworks.com/help/images/ref/houghpeaks.html?s_tid=doc_ta#examples .

So the Matlab version of imrotate seems to be more tolerant about the missing parameter "method" in this case, it just takes the given string "crop" to be an input for the next parameter "bbox".

This works in Matlab, but I think it is not documented in their help page of imrotate: http://de.mathworks.com/help/images/ref/imrotate.html .