Mayans Fail To Predict Mild Drought

At least, that's what scientists are thinking:

Relatively mild drought conditions may have been enough to cause the collapse of the Classic Maya civilisation, which flourished until about AD950 in what is now southern Mexico and Guatemala.

Scientists have long thought that severe drought caused its collapse.
But Mexican and British researchers now think that a sustained drop in rainfall of only 25-40% was enough to exhaust seasonal water supplies in the region.
The findings were published in the journal Science.
The research was conducted by the Yucatan Centre for Scientific Research in southern Mexico and the University of Southampton in the UK.
Scientists used advanced modelling techniques to estimate rainfall and evaporation rates between AD800 and 950, when the classic Maya civilisation went into sharp decline.
They found that a relatively modest decline in rainfall was enough to deplete freshwater storage systems in the Yucatan lowlands, where there are no rivers.

Nowadays, that's not that unusual of a year to vary by 25-40% from normal.  I didn't realize that area had no rivers.