In Mexican history, what is known as The December Mistake?

It was the December 1994 economic crisis.

The crisis was triggered by the devaluation of the peso in the early days of the presidency of Ernesto Zedillo. A week or so of intense currency crisis was stabilised when US President Bill Clinton decided to grant Mexico a loan to bail the country out.

http://www.nationmaster.com/encyclopedia/The-December-Mistake

The 1994 Economic Crisis in Mexico, widely known as the Mexican peso crisis, was triggered by the sudden devaluation of the Mexican peso in the early days of Ernesto Zedillo's presidency. The crisis is known in Spanish as el error de diciembre — The December Mistake— a term coined by the then ex-president Carlos Salinas de Gortari.

https://en.wikipedia.org/wiki/1994_economic_crisis_in_Mexico