For one, they're using some sort of link / browser history hijacking (something like
https://github.com/turbolinks/turbolinks, IIRC) which is buggy, for example:
* say you're on page A
* navigate to page B
* navigate to page C
* click your browser's back button, hoping to be back at page B
* the github UI screws up and keeps you on the same page while changing the URL in the address bar
* so you click back again
* now your on page A, completely skipping page B
* clicking forward takes you back to C
Additionally, GitHub is insanely slow. This page, for example, takes 3 seconds to load: https://github.com/Boemska/create-sas-app/commit/85224fe6622...
Couple broken history back/forward navigation with long page load times and GitHub is easily one of the most frustrating web sites to use.
Don't get me wrong, I'm not arguing that GitHub doesn't have value -- it most certainly does -- but that doesn't excuse bugginess and absurdly long page load times. If they fixed these problems it would be 100% awesome.