When Denny Duquette spooked and confused the audience of "Grey's Anatomy" earlier this season, it won't be the last time. He will appear again most probably on the 100th episode that airs next week (May 7), according to E! Online.
Denny died on the second season and re-appeared through Izzie's mind in the beginning of the fifth season when she began falling out of her cancer. His appearance in the future episode is not addressed further but creator Shonda Rhimes said, "Just a reminder: He's not a ghost, and we should all remember what his appearance means."
Denny's re-appearance is somewhat expected for during the 100th episode celebration on the set of "Grey's Anatomy" there photographed Jeffrey Dean Morgan who plays the character Denny. Additionally, on the official synopsis of the 100th episode, it is said that Izzie will experience "familiar symptoms".