I can only imagine the code that handles the previous and next buttons and the counter, with all the exceptions there probably are. the next of 403 is correctly 405, which correctly has 403 as its previous.

404 is handled exceptionally inside the sequence, and for a while it was double-exceptionally possible to land on it through the random button:

https://web.archive.org/web/20190328010454/https://plus.goog... (via explainkcd)