Adult students will occasionally bring English correspondence with them to lessons -

either their own in order to proof-read it, or what they have received in order to check their comprehension.  One student had brought in an email he’d received from a British colleague because he was amazed that he had found mistakes in it.  I’m still not sure if he was more surprised that his own English had become so good that he was able to find a native speaker’s mistakes, or if he was surprised that even native speakers make mistakes.