There has been an example with one of Suncor's ponds. There was a breach in the dam wall; it's been fixed since then. Worldwide, the type of dam that's used for tailings is ten times more likely to fail than a conventional hydro dam, let's say, so in general they are a risk.
The example I mentioned of the failure in Tennessee two years ago was a coal ash dike. Coal ash is one of the wastes they have in the U.S. from coal-fired power production. Its construction was quite similar to the tar sands tailings ponds, where it built up over time—higher and higher. One of the walls failed, and toxic sludge ran into the Emory River.