Compare http://en.wikipedia.org/w/api.php?action=query&format=jsonfm&generator=listpages&glspid=13 and

$.get( 'https://en.wikipedia.org/w/api.php?action=query&format=json&generator=listpages&glspid=13' ).then( function( data ) {
    for ( i in data.query.pages ) {
        console.log( i, data.query.pages[i] );
    }
} );

The first will order results by title. On Chrome, the second will order results by page id. This is due to Chrome's technically valid but annoying behavior of rearranging numeric JSON keys. Clients should ensure they do not depend on the browser's iteration order; this can be done by asking for and iterating the indexpageids parameter.

Noticed in the generator=listpages API calls, but any call using title parameter or a generator might be affected.