During our lectures on language and thought, I was absolutely amazed to see how the brain could know something yet be unable to speak about it, or somehow communicate its knowledge due to a severed corpus callosum. This idea of thoughts being unable to be spoken led me to think of stuttering.
Stuttering is a speech impediment that affects a lot people, mostly children. (I was a child stutterer, myself). It can range from the repetition of sounds, the prolongation of syllables, or elongated pauses between words and speech that occurs in spurts. For more information, go here. Many might recall the recent movie The King's Speech, where King George VI deals with many problems, such as his own stuttering.
Some scientists have found that while stuttering is often caused by emotional factors (such as stress or family dynamics), there are genetic factors as well (over 60% of stutterers claim someone in their family has stuttered as well). Stutterer's brains process speech and language differently than non-stutterers, but it's not that they don't use language and grammar properly. According to this article, when they begin to speak, their motor output does not function properly. In fact, "The right hemisphere is considered the non-dominant hemisphere for language, and the activity may indicate that the right hemisphere is compensating for something that is not happening in the left hemisphere". This subject still remains to be fully understood, but while most children phase out of stuttering by themselves, speech pathologists and therapists are able to help adults with their stuttering too.