Lost in Translation
Not an intentional con, but it worked out that way. I was working in Japan and I started dating a girl. We spoke English together because that was the language in which we were introduced. She never asked if I spoke Japanese.
Fast forward 18 months, we are getting engaged and about to have lunch with her parents. It was my first time meeting her parents and they spoke no English, and she was panicking cause she thought she would have to spend the whole time translating.
Lunch starts, I introduce myself in Japanese, and sit there chatting normally in Japanese with both her mother and father. I wondered why she was being quiet, and later she told me that she had always assumed I only spoke just a handful of Japanese.
She was rather upset with me for a bit. My future mother-in-law actually came to my defense and everything worked out. Story credit: Reddit / Tomerarenai