Diffing The New York Times on thermal paper (niemanlab.org)

Here's a longer write up on the hardware involved and thinking behind it. It's linked from the article but I thought this explained it better.

http://blog.nytlabs.com/2013/11/05/diff-a-live-comparator-fo...

Very similar to Berg's Little Printer

http://bergcloud.com/littleprinter/

Related project: http://www.newsdiffs.org