The Replication Crisis was the discovery that many fields of so-called science were producing experimental results that could not be replicated, because they were illusions resulting from bad statistical and experimental practices.

The replication crisis began in the early 2010s when several high-profile irreproducible results inspired mass replication attempts, revealing that the majority of papers checked in psychology and a number of other fields were not replicable. Some of the irreproducible results, like Priming, appeared to bear on rationality and were referenced in early LessWrong posts.

External Links:
Retraction Watch
Replication on gwern.net
Likelihood functions, p-values, and the replication crisis by Eliezer Yudkowsky
Wikipedia

Related Pages: Practice & Philosophy of Science, Psychology, Information Cascades, Falsifiability