We'll respond shortly.
Just your annual reminder that these services go down and will timeout your tests. Just download the kitten and serve it yourself, ya dummy.
It seems that postgres will sort consistently within a "page" of results. So the results on page 2 will always be the same.
But if there are duplicate values in your sort column and those duplicates cross a page boundary, it is entirely possible that one of those items will show up both pages.
So best to throw a secondary sort column in there if not sorting by a unique column.