I would highly suggest anyone who purchases a Sanguino to install a .1uF ceramic capacitor between Power and Ground on the board, it's really easy to do because on the Atmega644 DIP they're right next to each other.
I've attached documentation from Atmel, the manufacturer that would substantiate my suggestion.
A cap like the following would do the trick...
478-4855-ND
http://www.digikey.com/product-detail/en/SR201C104KAR/478-4855-ND/1550773
This cap should be placed, per the mfg, as close to the power and ground pins as possible. It's easy to solder them on the bottom of the board directly to the socket pins. Ignore the inductor in the picture, it helps but isn't as crucial.
From experience the Sanguino can be prone to stray fields and adding this capacitor is one method of improving this.
The attached documentation is provided from AVR040: EMC Design Considerations
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www.atmel.com]