One of the most dangerous trails on Oahu is Pu'u Manamana. The first time I tried it, I lost my nerve and turned back. The second time, with the Hawaiian Trail and Mountain Club, went better and I completed the hike. After that, I have hiked Manamana a number of times without mishap.
I have also written a detailed hike description of the Pu'u Manamana hike elsewhere. Check it out.
The photo at left provides some perspective of what this trail is like--spooky yet spectacular.