Mixing in the Milk
This is one of those questions related to Jewish law where, if the answer matters to you, it's going to matter a whole lot. At the same time, I'm sure plenty of readers are thinking, "What's the big deal?" They may never understand why it's a big deal to you. What's important here is that when you're a guest in someone's home, you respect their decisions on what to eat, while, hopefully, they respect yours as well.
If a host is going to attempt something as difficult and potentially divisive as a fleyshik potluck, they need to be absolutely 100 percent sure that all their guests are on the same page about the laws of keeping kosher. They also need to be prepared to scrutinize dishes that come through their door before serving them. It sounds like these hosts did neither of those things, and it was inconsiderate of them to put their guests, kashrut-observing or not, in such a situation.
When the dairy kugel arrived, the hosts could have said, "Thank you so much for bringing this. Unfortunately, dinner tonight is a meat meal, so we'll put this in the fridge for you to take home later." They could have added, "Kugels freeze beautifully, and we'd love to enjoy it with you another time." Or, they could have offered to serve it first (since dairy can be eaten before meat, but not vice versa), though this would have called more attention to the guests' mistake and perhaps caused unnecessary embarrassment. Regardless, the hosts should have graciously offered an apology for not making the "rules" of the dinner clear, even if they were absolutely positive that the fault was not theirs.
As for you, as with any potluck, if you have strict dietary needs, it helps to know who made what and what the standards are for their kitchens. If you had a few close friends there and could just stick with their food, that could have helped you feel more secure. You could have even just stuck to your own dish. I would hope that a kashrut slip-up wouldn't cause you to feel so uncomfortable that you had to leave, but if it did, you should have offered profuse apologies for leaving early and not told the hosts why. If, however, there was any possibility that the hosts didn't realize what had occurred, you could have pulled them aside and said, "I think so-and-so didn't realize this was a meat meal. Before people start eating, I think it would help to clarify what s/he brought and make sure it's O.K. to serve along with meat." Of course, if the hosts said they'd already made their decision, then your role as a guest would be to drop it.
Just to prove that no conversation is actually new, I have to share a related discussion that takes place in the Talmud. One opinion states, "A fowl may be placed upon the table together with cheese but may not be eaten with it." Just to clarify (ha!), the discussion goes on to say, "Of what table did they speak? Of the table upon which one eats; but on the table wheron the food is set out may without any hesitation place the one [food] beside the other." These rabbis might be talking about a potluck buffet! Just because some ancient rabbis had a similar question, my answer doesn't change, though perhaps this anecdote provides some compassion for the dilemma the hosts faced. (To give credit where it's due, thanks to Rabbi Helen Plotkin of Mekom Torah for introducing me to this text with the disclaimer that this is not to be regarded as any sort of endorsement or legal principle. This may be the first and last time Talmudic debate makes an appearance in this column! Whew!)