iTWire reports a dozen common or critical failure modes for an agile implementation, lifted from Agile Australia 2009. It's interesting to see the angle mainstream IT press in Australia has on this theme.
The last point in the article is one close to my heart. Falling back to traditional appproaches when it all gets too hard. As the agile point man at my current engagement, I definitely know that feeling.
On the other hand, we ran our second round of UAT last week and in both it and the first round we only came across one bug of moderate significance. This is a pretty fantastic result, especially from a team who's previous release didn't even run on day one of UAT, and that's not even considering the huge schedule differences.
It seems a process that enables, rather than tries to force particular agendas can really make a difference. Once the product is more mature we might see if we can turn out a white paper on the topic.