I find this a really interesting question. I’m sure I’ve heard of this theory before but I can’t remember where. I’ve noticed myself when giving discussion tasks on emotionally-engaging topics that students can make more mistakes. Generally speaking, when students are focusing on the message they’re putting across – rather than on the actual language – they make more mistakes, even in language they actually know. During such tasks, I’ve also heard students use a difficult phrase correctly and seen their joy in doing so.
I spent some time considering where you might find a reference to this theory or research and I can’t think where I’ve seen it. Pilgrims school and series of books cover this topic well (for names of authors, look at their site under teacher training). Piaget and Julian Edge wrote about errors. You could start with any of these authors. If you find it, please write again to let us know. I’d love to find out who it was who said this. If I come across the name or piece of research, I’ll let you know.