It does happen, and i'm afraid we often don't notice/realise.
Yep, i have seen the reverse, a 3 year old running around quite happily but didn't speak.
Case studies can produce some wonderful (valid) results.
DNA has the ability to produce more variables than there are stars in the universe, (i try to see them and often fail).
I once saw a group study of people that had voices in their heads continually criticising them, curiously, the one thing they all agreed on was the problem was better than the drugs which (prescribed) took the voices away, (because the drugs made them empty nobodies).
Can i feel like a failure at times, Yes ; because i cannot help such people.
Sorry if i went a bit extreme to illustrate a point. ( i enjoy the discussions on this forum from a fine group of individuals).