If you are invited over to friends' home for dinner...
I usually bring wine. I once did and had a (not close)friend get really angry at me for not calling to see what they were serving, as I brought red and they were having chicken. The same friend would, if he'd invited me at the last minute (an hour before, while I as at work) and I asked if I could bring anything, he'd give me a list of specific ingredients (cilantro, naan, a specific, hard-to-find brand of mango chutney, organic vanilla ice-cream and blackberries) plus kingfisher beer. It's a lot
of shopping for a last minute dinner at someone else's house. I have another friend who asks for specifics plus wine -- she had me pick up $20 worth of takeout fried rice she'd ordered
because her husband wants rice with th meal and she didn't have time to cook rice (and she knew he'd want an extra order for his work lunch)... since this happens regularly (the last time I as invited for dinner I was asked to go in halves for pizza when I got there, plus the wine and specific dessert she'd asked for). I gave her the bag with the bill stapled to it... and she said "thanks for contributing!" It wasn't a potluck or a big dinner party, just a normal evening meal I'd been invited to an hour before. Don't get me wrong, I'm happy to contribute -- but I always thought it was pretty bad manners to demand really detailed specifics. Interestingly, the same friend won't take just bring yourself for an answer when i invite her to my house for dinner, she's insisted on bringing a dish -- (an appetizer that didn't really work with the meal, but I didn't tell her that -- and then showed up without it, saying she'd run out of time (which worked out for the best).