A frequent argument in our local AL community arises because a couple of DM's have decided that rogues can't make 2 sneak attacks in a round, one on their turn and another with their AoO using their reaction. Even though the RAW ability states that it is once per turn, and there is a RAI ruling in the sage advice pdf, they insist that it's a once per round ability.
Another one from AL: Some magic staffs say that they can be wielded as a magic quarterstaff. Shillelagh lets you use your spellcasting ability instead of strength when wielding a quarterstaff. The spell text says "The weapon becomes magical if it isn't already," so clearly it can be cast on a magic quarterstaff. I and some other players have PC's - druids, clerics and warlocks - that have acquired magic staffs with features that trigger on a melee hit (eg Staff of Withering) and want to use Shillelagh in order to use them in melee. So far DM's have been perplexed by this and on the spot ruled that if a magic staff has Shillelagh cast on it, you can't use its other magical qualities. It can be a Shillelagh or a Staff of Thunder and Lightning but not both at the same time. When we point out that the text of the cantrip allows it to be cast on a magical quarterstaff and what would be the point of that if it negated the qualities of the magic item, they are not convinced. As a player, these sorts of rulings are really frustrating when they are about things that are decidedly not gamebreaking. We have raging barbarians in the group who can carve up a hill giant in 2-3 rounds; why does me doing an extra 2d10 damage 3x per day with a Staff of Withering worry you so much that you want my attack mod to be -1 instead of +4? This is a case where the fluff in the spell description confuses the simple explanation of what the spell actually does. The DM's are looking at the "imbued with nature's power" part and deciding that this power temporarily replaces the magic power in the staff. If the spell description had just skipped the first sentence, I don't think we would be having this argument.
On the flip side, DM's don't seem to particularly care about what PC's are holding when they cast spells with somatic or material components. In fairness, I think this is something that players should be monitoring as part of roleplaying how their PC's interact with their environment.
Ammunition: You can use a ranged weapon that has the ammunition property ...only if you have ammunition. Each time you attack with the weapon, you expend one piece of ammunition...At the end of a battle, you can recover half of your expended ammunition by taking a minute to search the battlefield. Nobody seems to be keeping track of ammunition in AL or anywhere else. Melee fighters with great swords don't get to ignore the two-handed property. Why are ranged fighters ignoring the ammunition property? While every other player at the table is trying to manage limited resources of spell slots, rages, smites, ki points, etc (well not the rogues - they are just awesome that way) the Sharpshooters don't seem to be running out of arrows, even in places where the missed shots would not be recoverable, like open water or fields of tall grass. Once again, I don't think this is the DM's job any more than it is their job to track spell slots. Players who decided to play a PC who uses ranged weapon attacks are making an implied commitment to the rest of the players and the DM to roleplay their character's use of their resources, just like all the other players. Having less and less of what you need as you get closer and closer to the BBEG is one of the defining elements of D&D play. I'm reluctant to mention this in the games I play because as far as I know I am the only one who cares.