This has happened to British Agents before in WWII. I remember seeing a documentary about training British agents to infiltrate France. While in France one agent got caught out by asking for a black coffee. This was the default coffee at the time in France, people would just ask for a coffee and expect a black coffee. This made him stand out.

There are similar stories like this in movies, such as the Great Escape, when one of the escaping prisoners is dressed as a civilian and is boarding a bus in town. A German Officer says in English "Good luck", the prisoner replies in English "thank you" and is caught.