The Rothman Disintegration - S5-E17
Corrected entry: After Sheldon has told Barry about the rules of Rock/Paper/Scissors/Lizard/Spock for the first time, Barry then replies "I'm sorry, can you repeat that." When he says "repeat", he correctly pronounces the "r." However, it has already been established throughout the series that Barry pronounces all his Rs like Ws due to his speech impediment.
Correction: Barry doesn't always mispronounce the letter R. It can happen with certain Rs and Ls.