It appears that three tourists climbed over the barriers at the top of Vernal Falls in Yosemite National Park, apparently to get a better photo, all three slipped and fell into the river, and went over the edge in front of a crowd of horrified onlookers.  So far, their bodies have not been found.  Here is a picture of the edge of the falls and the viewing area. 

Here is Vernal Falls in full spring runoff mode.  That's an obvious danger, especially standing right at the edge.

However, having been to the top of Vernal Falls myself, and having seen this area, there is a danger that is not obvious, and which I believe explains how three people could all go into the river and over.  The rock here is granite, and normally granite is rough and provides a great grip to hands or shoes.  In contrast to that, next to the river, the rushing water polishes the granite to a mirror smoothness, and if that polished surface get wet to any degree, it becomes unbelievably slippery.  I speak from experience here.

Further, the change from a rough grippy surface to a super slick one occurs in the space of just a few inches, and there is no obvious change in appearance to let you know.  In this case, these folks likely wanted to get a better, more dangerous looking picture, and being young, bold and a bit foolish, decided to climb the barrier and stand close to the dramatically rushing river.  They almost certainly unknowingly got onto the slippery and wet granite just next to the river, and when one fell in and the others started to scramble to help, they all went in one after the other.

Another danger from this river polished granite is that once you go in, it gives virtually no purchase to your hands or feet, so it is very hard to climb out, or sometimes to even stand up while you are being pushed by the fast moving water.  Image trying to climb up a wet, steep mirror. These folks, being so close to the edge, just didn't have time to even make an much of an effort to stop themselves.

Just up the river a short distance from the edge is a large swimming hole, and when I was there several years ago, probably fifteen people were swimming in it.  Even that is kind of a crazy risk, to my eyes at least.

If you are ever in the granite country of the Sierras, keep this in mind if you are around fast moving water.