The actor admits he wasn't thinking about the impact of what he was doing when he tweeted shots of Eric Dane on the set.
Dane's character, Mark Sloane, was left for dead at the end of the last season of the medical drama.
In a pre-taped interview with chat show host Ellen DeGeneres, which will air this week (beg10Sep12), Dempsey explains, "I tweeted some pictures of this season and they were like a spoiler alert. All of the sudden I got all these phone calls from (the network) ABC, like, 'You have to take this picture down!'
"I was just enthusiastic: 'This is a great image, I should shoot this.'"
But the damage was already done before Dempsey removed the shot of Dane: "Now we know he survives and comes back and he was just napping in between takes."
But Dane is planning an exit from the show - earlier this summer (12), the actor announced the upcoming ninth season of the show will be his last. The new season of Grey's Anatomy is set to debut in the U.S. later this month (Sep12).