I once convinced the lads at work I could speak French as we were getting a new French Project Manager. They got me to speak French so I came out with a load of random words I remembered from school. They were convinced but when she started, they got me to repeat it to her.
Instead of owning up I rattled it off again. She looked a bit confused then laughed and told everyone I had just said "The dog is under the library and he is wearing a hat."