There are rules in polyamory that are like training wheels. They're helpful when you're starting out, but once you have the hang of multiple love relationships, they create problems rather than solve them. Consider what I call the Symbol Over Substance mistake. (I'll use male pronouns, but gender is irrelevant here.)
Here's a previously monogamous couple, Chris and Pat, who are shifting into polyamory. Pat says, "Okay, I have some fears, but I'm cool with us fucking other people. But you must never, ever sleep overnight with anyone else. That is something that's special to our relationship." Chris doesn't think it's such a big deal, but he agrees to please Pat.
Time rolls on. Chris and Pat develop other relationships, but Chris continues to love Pat and everything is rosy. And then one night, Chris lies down on the bed beside his other partner to cuddle for a little while before leaving—and falls asleep. He wakes up at dawn.
One of two things can now happen. Pat could recognize that nothing has really changed in his relationship with Chris, and that this incident means nothing except that Chris was tired. Ideally, he'd even realize that, gee, maybe Chris should just start staying over at his sweetie's house sometimes rather than driving home half awake at 4:30 a.m.
Unfortunately, the more common reaction is Pat loses his shit. He has been betrayed. His whole relationship with Chris is a hollow sham and his heart is broken, all because Chris fell asleep.
I believe in honoring poly boundaries, but my sympathy is mainly with Chris. Pat's emotional crisis is of his own creation. He took an arbitrary symbol—"Chris sleeps with only me"—and gave that one symbol a lot of power. He made it the solitary litmus test of whether his relationship with Chris was stable and safe. People do this because it's simpler than having to really examine themselves and their feelings. It's basically replacing sexual monogamy with some other symbol. But as long as you assign power to symbolism rather than what's real, then you're mistaking the form of love for the substance. Sleeping with Pat is not what makes Chris love him and treat him as special. It's because he loves him that he sleeps with him.
If you doubt me, turn it around and look at it. Chris could sleep with no one but Pat, and he could still treat Pat terribly in other ways and trash their relationship.
I said my sympathy was mainly with Chris—but only mainly, because he bears the responsibility for not saying, "Okay, I agree for now, but in six months let's revisit it and see if we still both think it's important. Because no matter where I fall asleep, I love you, and the way I feel about you is what's really special to me." One of the scary things about polyamory is that you have to keep examining things. Is what we're doing working? What needs to be changed? Breaking a rule is a bad way to start that process. As with riding a bike, you have to look at what's ahead to stay safe.