This is discussed in Brian Christian's The Most Human Human (a book about participating in a Turing test contest); there was a chatbot that claimed to be a young person from another country and made a lot of mistakes. People found it pretty convincing and forgave some of its linguistic comprehension errors because they had a plausible explanation for why it wouldn't be good at English.

Sounds like "Eugene Goostman". It's common to "cheat" on the Turing test by pretending to be an incomprehensible human, like "PARRY" which "simulated" a paranoid schizophrenic, about which Daniel Dennett relates this anecdote in his essay "Can Machines Think?":

"My favorite commentary on it was Joseph Weizenbaum’s; in a letter to the Communications of the Association of Computing Machinery (Weizenbaum, 1974, p. 543), he said that, inspired by Colby, he has designed an even better program, which passed the same test. His also had the virtue of being a very inexpensive program, in these times of tight money. In fact you didn’t even need a computer for it. All you needed was an electric typewriter. His program modeled infant autism. And the transcripts--you type in your questions, and the thing just sits there and hums--cannot be distinguished by experts from transcripts of real conversations with infantile autistic patients."

It's well worth a read!

http://www.nyu.edu/gsas/dept/philo/courses/mindsandmachines/...

Eugene Goostman is exactly right -- and apparently genuinely fooled people.

Thanks for the reference.

Edit: It's very interesting and Dennett anticipates a number of Christian's observations, even in the part of the paper written before Dennett was associated with the Loebner contest.