The start of the article infers that the London Whale Trade was caused by manual handling of data between spreadsheets.

The real reason came down to a flaw in the formula they were using. From the JP Morgan report:

    ... a decision was made to stop using the Basel II.5 model and not to rely on it 
    for purposes of reporting CIO VaR in the Firm’s first-quarter Form 10-Q. 

    Following that decision, further errors were discovered in the Basel II.5 model, 
    including, most significantly, an operational error in the calculation of the 
    relative changes in hazard rates and correlation estimates. 

    *Specifically, after subtracting the old rate from the new rate, the spreadsheet 
    divided by their sum instead of their average, as the modeler had intended.* 

    This error likely had the effect of muting volatility by a factor of two and 
    of lowering the VaR.... It also remains unclear when this error was 
    introduced in the calculation.

Source: http://www.zerohedge.com/news/2013-02-12/how-rookie-excel-er...