The thing I mess up on regularly are monster effects that happen on the players' turns. . . I keep wanting them to all happen on the monster's turn, but that gets just as confusing. So recently, a troll variant I was running had a gibbering confusion effect on anyone entering into or starting their turn within 20' of it. I kept wanting to force people to make saves on the troll's turn when he got within 20' of them and that led to actual confusion within a round.
While I understand 5E's attempt to make this kind of thing consistent, my brain keeps wanting for a monster's effect to be checked on in the monster's turn - because as DM it is easier for me remember that way. Except half the time I ask PCs to also make the saves on their own turn when they move into the radius.