Let's say you've been with your lover for a while. You know their body. You know how they like to communicate. You know their preferences for food. You know what types of clothes they like. You know a vast plethora of trivial, and not so trivial, details about this person you love. Now, imagine that you have this in triplicate, except that a lot of those details differ from person to person.
Russell and I were in bed the other day, and he did this thing that one of his other partners really likes. For me, it's kind of uncomfortable, and not sexy fun-making. I flagged it playfully by saying, "Wrong chick!", and he said, looking puzzled and repeating the action, "No...you really like...Oh shit!" We got a good giggle about it, but this isn't the first, nor last time, something like this is likely to happen, for either of us.
From buying the preferred beverage and presenting it to the incorrect partner, to a caress that makes one person giggle, another collapse in a puddle of goo, and triggers a third to lash out, there are so many things to track between partners, that cutting everyone some slack on these types of mental clerical errors is a merciful skill to learn in poly.
Particularly when transitioning between partners in relatively rapid sequence, it's easy to get muddled. Take a moment or two when shifting to remind yourself of who you are with NOW. Allow yourself to ask if you're having trouble pulling up relevant data about personal preferences.
If you have a partner that has a difficult time moving between relationships cleanly, in regards to recall, or touch, or whatever is important to you, let them know! Don't patiently endure a sex act that your metamour would be thrilled to receive that leaves you bored. Don't eat the spicy food that your metamour enjoys which will make you uncomfortable the rest of the evening out of politeness. Ask them to change the movie selection to a genre that you actually enjoy, and to please see the originally offered option with their other partner.
Being poly isn't a free pass to be unobservant, or treat everyone the same, just so you don't screw up. It adds another layer of complexity and memory that one needs to be mindful of. When you mess up, own it, apologize, and recommit to honoring the complexity and individuality of those you love. We're all fallible individuals muddling through this together. Stay compassionate with each other, even in the face of an "Ooops!".