They can be joyous events, these gatherings of friends and food called potlucks. You know, a genial host and happy guests each toting a dish that perfectly complements salads, entrees and lip-smacking desserts spread across a flower-bedecked table.

But they can get ugly — fast — especially when your BFF disregards her promise to bring cherry pie for dessert and shows up with a seven-layer salad. Meanwhile, the guy who was supposed to bring buns for a couple dozen brats also brings a seven-layer salad.

And just when it seems things can’t get any worse, the first guest to hit the food table is the one who brought along a plastic take-home container to fill up with goodies.

We’ve got nothing against seven-layer salads. Honest. And taking leftovers home is fine — when you have the host’s blessing.

“There’s a good side to potlucks in which everyone feels like they’re bringing something and they get to bring something that they like,” says Lizzie Post, author and spokeswoman with the Emily Post Institute in Burlington, Vt.

Then there’s the other side.

“Some people get invited to a potluck and think ‘Great you’re inviting me to a party and I have to do the cooking.’ I’ve seen people fall on different sides of that line depending on how their day is,” says Post. “And I’ve heard some get really frustrated because when you talk about being a reciprocal host, they feel it’s not fair that they throw a dinner party and serve filet … then they get invited to a potluck. They kind of feel, ‘Great, it doesn’t really match what I gave you.’ ”

What fuels this annoying behavior may have little to do with the food, says Samuel Gladding, a professor who chairs the department of counseling at Wake Forest University in Winston-Salem, N.C.

“Part of it may be a little rebellion, especially if you’re told what to bring,” he says. “Part of it might be a little passive-aggressive anger because maybe you wanted to go the beach and not to the potluck function. Part of it is … that we all want some control over our lives and again, if I am bringing potato salad instead of chocolate cake, then I’m saying ‘I have control of what I’m doing. I know you might have said chocolate cake. I think what people really need is potato salad.’

“And part of it is just human folly in terms of forgetting what I should have done or needed to do or I was asked to do.

“We do expect people to be sensitive to the situations that are before them,” says Gladding, “and doing something like we’re talking about does show a lack of sensitivity, it shows a lack of preparation and it actually in many ways shows a lack of respect for everyone who is invited to this.”

Post advises hosts to be sensitive to people’s strengths and/or limitations. And for guests, she says, “Be flexible. Understand your absolute favorite thing to bring to a potluck might not be the thing that is needed.”

And both host or guest should also remember, Post says: “You’re given permission to say no.”

On the other hand, it also pays to lighten up: “A lot of etiquette is to roll with it in the moment as long as it’s not something that’s really degrading,” she says. “You can always decide later if you want to repeat the experience.”