This story happened several years ago, when I was in college, but I've never forgotten the graciousness of my guest. I'd made a good friend while studying abroad and we hadn't seen each other in several weeks, so I sent her an email asking her to catch up over dinner at my place. We'd been traveling in an Eastern European country and eaten lots of meat-heavy meals together (Eastern Europe!), so I bought ground lamb and fixings and made lamb burgers, because I love lamb and knew I'd seen her eat it.
I'm not the greatest cook -- never have been, and still not much better than adequate -- but goodness knows I tried with those burgers. They came out a bit greasy and heavy, but largely edible. I might have served a salad or something as well, but the lamb was definitely the main focus of the meal.
She came over and we had a very nice time. She complimented my cooking (lies! such welcome, pleasant lies!) multiple times and ate every bite, and we had a terrific time catching up. She even sent a thank-you email thanking me again for the delicious meal.
About a month later, we went out to an Indian restaurant, and I noticed she was eating basically rice and vegetables in sauce. I told her her entree looked very nice, and she gave me a lovely smile and said she'd been vegan for the past few months for health reasons, and was enjoying her new diet. I didn't have to think too long about that math -- yes, my friend, while on a strict vegan diet, ate a ground-lamb burger, rather than embarrass me or let me feel uncomfortable as a hostess! I certainly wouldn't expect anyone to make that sacrifice (we could have ordered vegan Chinese food! I understand the importance of guest's dietary restrictions), but every time I see her, I think about what a gracious dinner guest she was.
(Also, I don't remember the specifics anymore, but when I got home, I scoured the email chain with my invitation to see if she'd told me she was eating vegan, but she hadn't. I don't remember why she didn't mention her vegan diet when I first invited her, but if you can take my word for it, I do remember thinking there was a pretty good explanation of the miscommunication.)