There are a number of spell instances, such as pretty much anything having to do with Illusion (Minor Illusion is particularly bad with this), plus a few other notable cantrips... Playing an Illusionist is only as powerful or useless as your DM decides it is at any given moment.
Whether or not an improvised weapon is actually an improvised weapon and thereby uses (and can benefit from) the Tavern Brawler feat, or if it's suitably weapon-like that you can just use it with your normal proficiency.
Is your lie "dramatic" enough that you can use Performance in place of Deception? When isn't it?
Tools.
There's something niggling (it's a word, I swear) in the back of my head about some other rule that's rather open to interpretation, but those were the things that came to mind offhand. Even just spells have a number of weird and/or muddy workings.
Some things are also just poorly described, such as how all of magic missile's darts strike simultaneously, meaning that an evoker wizard can add his INT mod to each dart... But a dragonblood sorceror can only add his CHA mod to one scorching ray, for example. Or that "nets should always be thrown with disadvantage", instead of giving it a minimum range of "ranged weapons are used at a disadvantage in melee" and a max range of "ranged weapons are used at a disadvantage when firing at long range" without actually mentioning that "yeah, we knew what we were doing, and this is how it works". Or how the crossbow expert skill officially regards the hand crossbow itself as "a one-handed weapon", meaning you can fire a single hand crossbow once and then fire the same one again as your bonus action... But if you're wielding something else in the other hand, you can't load it for a second shot, even though you're "ignoring the loading quality".
Except you can, because you can drop the other weapon as a "freer-than-free" action, reload as a non-action, and then pick up the weapon again as your turn's free action. Unless the DM says you can't because that's silly, of course.