According to Guinness World Records (2008), the highest single-turn Scrabble score in a real game is 392 points achieved by Karl Khoshnaw (UK) on 11 April 1982 when he played the nine-timer CAZIQUES spanning two triple word squares.
"High Single-Turn Scrabble Scores,"
Word Ways: Vol. 46
, Article 3.
Available at: http://digitalcommons.butler.edu/wordways/vol46/iss3/3