While Ron Howard and Cindy Williams are both in the movie, they don't play a couple, and their characters aren't mentally challenged.
* Ron Howard plays "Steve Bolander", a young man who is struggling with his future and his feelings for his girlfriend.
* Cindy Williams plays "Laurie Henderson", Steve's high school sweetheart.
The line "You left the oven on!" is indeed said by Cindy Williams' character to Ron Howard's character. However, it's not in a romantic context.