Not really anything that has gotten off the ground.

The problem is that we'd probably never know if there was one since the North Korean government is so good at suppressing information. A few years ago, there were reports of a coup attempt against Kim Jong-un while he was out of Pyongyang - reports of gunfire in the streets and a whole military unit attempting to overthrow the government, but it never went anywhere. Part of the "evidence" against Jang Song-thaek, Kim's uncle that was executed, was his "anti-regime activities," implying that he was working to overthrow Kim. But all of this is recent.

A North Korean defector tells of two attempts against Kim Jong-il when he was the leader. I've read about one attempt before and I'm not sure if it's the same one he's talking about here. I had read that a would-be assassin came up to Kim and attempted to fire a gun at him point blank but it jammed and he was killed on the spot. This defector mentions someone attempting to use an automatic weapon to kill Kim but he was captured before he could. It might be the same incident, but I'm not sure. The second incident was a truck driver attempting to ram Kim's car. Attempts on the regime itself are also mentioned but again, there is no solid evidence. Everything is circumstantial.

A former CIA official discusses some coup attempts that the defector above mentioned as well - an attempt to remove Kim Il-sung in the early 90's by a group of army officers and an attempt against Kim Jong-il by an army unit in the mid 90's, both of which failed and resulted in numerous executions and imprisonments.

There have been some other possibilities, the most notable being the Ryongchon disaster in 2004. A massive explosion at the train station which destroyed the town and killed or injured up to 3,000 people (no one really knows how many). Kim Jong-il had passed through the station earlier on his way back from China and there was speculation that it was an assassination attempt. However, newer thought on the explosion is that, since Kim had traveled through the station on his special train, the schedules and tracks were not the same, and it was simply an accident due to that.

But as far as any real, legitimate coup attempts that have really been set in motion, no. Everything has been caught early and crushed.