A very good post on this by Matt Strassler from earlier this year:<p><a href="http://profmattstrassler.com/articles-and-posts/particle-physics-basics/neutrinos/neutrinos-faster-than-light/opera-what-went-wrong/" rel="nofollow">http://profmattstrassler.com/articles-and-posts/particle-phy...</a><p>There were apparently two problems:<p>1. An improperly calibrated clock
2. A fiber optic cable was not fully plugged in. This led to a little bit of light leaking out from the cable, which lead to a (very small) delay in recognition of the arrival of a pulse on the fiber.<p>Yes, modern physics experiments are pretty complicated and lots of things can go wrong!