Table manners
Context: Before my college semester abroad in London, each person was required to present on an aspect of British Life, so that we would be adequately prepared when we finally met what my trip leader kept describing as "Real British People" (RBPs). My topic was table manners. Rather than do any formal research, I called up my mother (an RBP in her own right) and wrote down everything she said.
For some reason everyone seemed to be more offended than grateful for the information...