If you miss a word in a conversation, you automatically fill it in. This is usually pretty reliable even with unfamiliar material, since if you follow standard grammar rules there's a decent amount of redundancy. Sometimes, though, this doesn't work. You think you were paying attention, but you missed parts and filled them in with what you wanted to hear.

Ideally there is a back and forth that checks that important information was correctly passed on. Failing that, some redundancy can make a big difference.