I've managed to not-really-learn several languages over the years. On the whole, it's a good thing, but there's one major problem. My brain has never quite managed to sort them into "English" and "not English". As a result, I have a bad habit of peppering otherwise normal conversations with snippets of foreign languages, mostly German.
Occasionally, it sidetracks class. Today, trying to get a student who was very quiet to speak up, I asked, "Was ist es?"
"You spoke German."
"No one here speaks German."
"Well I do, a little."
"Well it's creepy!"