When I was in high school, my first "real" girlfriend was from a devoutly Jewish family. At the time, I thought I was pretty cool, and had a 5-day scruff going -- I looked terrible, but it's relevant to the story.

She invites me to Passover Seder at her house, with her whole family. When I arrive, I put the yarmulke on to fit in and am having a fine time dipping my challah in honey.

After the whole 'Why is this night different?' spiel, her grandpa says to me jokingly "Nannerlmozart, you look like a proper member of the tribe with that beard and yarmulke! Are you sure you're not Jewish?" and I reply "Haha, no, I'm of German descent but not Jewish, as far as I know!" and he asks, in a slightly more serious tone: "Really? I hope you weren't on the wrong side." I idiotically responded with the truth: "Unfortunately, my family was but they didn't have much of a choice."

That killed the jovial atmosphere of that seder. None of her grandparents talked to me at all after that, and I could tell the rest of the family only acknowledged me so that my girlfriend didn't feel like shit. One of those time-and-place things that taught me that honesty is not always the best fucking policy.