I've been playing and DMing since the 80s. I've been DMing and playing 5e for a number of years now. I have read the 5e core books cover to cover. I have authored published D&D (OGL) material. I am more than a year into my latest 5e weekly campaign as a DM.
There are 5e mistakes I find myself making again and again.
I realized after my last game session that I keep taking zombies off the combat tracker in fantasy grounds immediately after they go down. I do this with most monsters when they die to keep things less cluttered and distracting and to make things more manageable, but with zombies this means one of their 5e signature features, saving to pop back up the round after dying, does not come into play. I kept forgetting this zombie power in the moment in face to face games as well. Zombies showed up a bunch when running the gothic horror Carrion Crown adventure path converted to 5e, including an
add on zombie apocalypse scenario adventure. I have yet to remember to check whether a 5e zombie pops up.
Multiattack. I keep forgetting to look and see if there is a separate entry in actions for multiattack. Thugs having half their normal attacks are about half the mechanical threat they normally would be.
What the warforged PC is and is not immune/resistant to. Ghoul paralysis works on robots? They are half damage resistant not immune to poison?
Most of this works fine in the moment and is mostly invisible to the PC side, but I see it and missed opportunities on cool things like evocative monster mechanics is a bit disappointing.
As a player, my last wizard PC I kept forgetting I had shield. A great reaction spell to block a hit, if you remember to use it.
Any small things that seem to keep tripping you up?