Or when replies to a now removed comment ended up as top level comment, making reading the comments sometimes super confusing. Reddit's backend is probably nothing but spaghetti code. Hence why the redesign & mobile app are both also such piles of garbage. It would likely need a complete rewrite, but they probably don't have any sort of way to do that and maintain all the old legacy content. That's also a good reason to go closed source, especially if you want to become a traded company. Investors are probably not having the hots for garbage code that everyone can look at & criticize.