Lack of uniformity in halakhic standards can often introduce (sometimes unforeseen) complications into social situations. For instance, a guest's kashrut standards may not be the same as his host's. The 18th Century German halakhic authority Hagaon Rav Yaakov Etlinger, zt"l, the author of Responsa Binyan Tzion, was asked the following question, and his response is quite relevant to contemporary Orthodoxy:

Question:
One person is hosting a guest who is stringent about a certain halakha. The host himself believes that the guest's chumra (stringency) has no legitimate basis in halakha. Both the host and the guest are halakhically knowledgeable. Is it permissible for him to feed his guest food that his guest believes is forbidden? Response: The Binyan Tzion's discussion centers around the following two Talmudic halakhic anecdotes about pairs of Amoraim on different sides of a halakhic debate.

A. Chullin 111
Rav forbids eating fish (or any parve food) cooked in a meat utensil along with dairy; Shmuel permits it. Rabbi Elazar (a student of Rav ) was visiting Shmuel. They were serving a meal at Shmuel's house. One course was fish cooked in a meat utensil. Shmuel was fed this along with a dairy condiment (kutach). When Rabbi Elazar was offered the fish with dairy he refused it. Shmuel said to Rabbi Elazar, "I gave it to your rebbe (Rav) and he ate it." After this incident, when he visited his rebbe, Rabbi Elazar asked Rav if eating at Shmuel's house was evidence that he had retracted his opinion about the fish. Rav responded, "G-d forbid that the child of Abba son of Abba (Shmuel's father, a great saint and sage) should feed me something that I believe is forbidden."

B. Sukka 10
Rav Nachman argued with Rav Chisda and Rabba son of Rav Huna about whether sukka decorations (a colorful cloth spread out under the skhakh) hung more than 4 handbreadths below the skhakh invalidates a sukka. Rav Nachman held that such a sukka is kosher and Rav Chisda and Rabba son of Rav Huna held it is invalid. Rav Chisda and Rava son of Rav Huna (who invalidate it) went to visit the exilarch during Sukkot and were hosted by Rav Nachman. He hosted them in a sukka whose decorations hung below 4 handbreadths (thereby invalidating it, according to the two scholars). They did not make any comment on the matter. When Rav Nachman approached them, asking, "Have you retracted your opinion?" they replied, "Because we are on the way to do a mitzva we are actually exempt from the mitzva of sukka altogether."

The Ritva's comments
The Ritva quotes an opinion that infers from the episode in Sukka the following principle. If a host is a halakhic authority and rules leniently about a certain issue, he can put his guests in a situation where they will, unbeknownst to them, follow that leniency they do not subscribe to. Rav Nachman, points out this opinion, had the two guests sleep in the sukka even though the guests themselves believed it to be invalid.

The Ritva himself rejects this. Having a guest unknowingly eat food or do something that the guest believes is prohibited because of "putting a stumbling block before a blind man" (not facilitating another's sin). The story in Sukka is no evidence against this rule. First of all, in the story in Sukka, the guests were able to see how the sukka was constructed. They could have chosen not to sleep in the sukka if they wanted to. Secondly, in the Chullin episode, Rav said emphatically, "G-d forbid that the child of Abba son of Abba (Shmuel's father, a great saint and sage) should feed me something that I believe is forbidden." Rav clearly holds that such behavior is prohibited. The Ritva also cites his rebbe (Rav Aharon Halevi zt"l of Barcelona) who ruled likewise.

The Binyan Tzion's additions
After the Ritva's comments, we are still left with some questions on the plot of both episodes. Why did Rav Nachman wait until the morning to ask the other rabbis if they had retracted their opinions? Shmuel's behavior also seems strange. Why did he try to feed Rav Elazar the fish?

To resolve the difficulty, the Binyan Tzion assumes that the episode in Chullin is slightly more complicated than previously assumed. Rav Elazar (the student of Rav) was unaware of a subtle distinction within Rav's opinion. Rav only prohibits eating fish cooked in a meat vessel along with dairy if there is still some taste of meat in the fish. If there is no meat taste in the fish, even Rav agrees that it can be eaten with dairy. Shmuel offered Rav Elazar fish that was really permitted to Rav Elazar, but Rav Elazar did not realize it.

The sukka episode should be taken at face value, though. It is likely that the two rabbis did not see that the sukka was invalid (according to their own opinion), but Rav Nachman, says the Binyan Tzion, still had them sleep there. He might not have viewed such behavior as "lifnei iveir" (this is how the opinion the Ritva quoted learned the Gemara). He therefore only questioned them in the morning when they had already seen the sukka, yet remained in it.

However, Rav clearly considered feeding someone something the recipient believes (mistakenly, according to the giver) is prohibited, and we rule like Rav, despite Rav Nachman possibly arguing. Therefore he accepts the Ritva's and his teacher's ruling even though he argues on some of the details of the case. The Or Zarua (section 603) also seems to rule that a lenient host cannot feed a (in his opinion misguided) stringent guest without informing him first.

Practical Conclusions
What is proper guidance for a guest who holds to a stringency that his host does not? He should (based on the Or Zarua 603) ask his host (tactfully!), just like the Gemara concludes concerning Beit Hillel and Beit Shamai, who argued on many halakhic issues but retained social connections. If he cannot ask, it is legitimate to assume that he will not be fed what he believes is prohibited even though his host does not.