What cultural faux pas should I try to avoid making in Wash DC?

We asked people in Wash DC if they could share any humorous cultural blunders they commited. For new expats, keep in mind that these incidents are an inevitable part of expat life. Learning to laugh about them is the key!...

Other Questions:

•  What do I need to know before moving to Wash DC?
•  How do I find a place to live in Wash DC?
•  What is a typical expat home or apartment like in Wash DC?
•  What is the average cost of housing in Wash DC?
•  Should I buy or rent a home in Wash DC?
•  What should I pack when moving to Wash DC?
•  What cultural faux pas should I try to avoid making in Wash DC?
•  Why do people move to Wash DC?
•  What are healthcare services like in Wash DC?
•  What are medical services in Wash DC like?
•  What are typical rents in Wash DC?
•  What appliances are typically included in a rental?