Are we counting rules that so many people just get wrong that it's almost like a common set of house rules?
- I've seen a lot of DMs allow PCs to ready a full attack, or ready a spell without casting and holding it and requiring concentration (actually, I do this latter one, too, as an explicit house rule).
- I've seen lots of players cast healing word (bonus action) followed by a non-cantrip spell, and nobody ever calls them on it.
- I've seen many groups that allow Inspiration to be spent after you roll, as a kind of retroactive advantage.
- I've seen so much confusion over two-weapon fighting and bonus actions (i.e., a PC makes a bonus-action off-hand attack and doesn't realize its a bonus action and tries to take another bonus action on the same round) both from players and DMs that I wonder if it's intentionally confusing.
- Every group I've ever gamed with has had at least a few people who thought that 20 was auto-success and 1 was auto-fail on skill checks and saving throws. (Personally, I think this works better and should be the official rule.)