I don't know whether these are from the back office machine or the public machine. I suspect the latter, although customers' ability to 'pull' the strip to create errors was supposed to have been eliminated earlier so I can only assume this was a machine fault (see comments, several people have confirmed the fault) - and a spectacular one at that, with the printing spread over 8 stamps instead of 6:
US collectors would categorise these as 'Errors Freaks or Oddities'. Thanks to Greg for these and other similar pictures.
UPDATE 8 January: A video of the Cattle stamps being dispensed at Stampex has now been loaded to YouTube. The strips weren't actually held up to camera, probably for fear of alerting Royal Mail staff, but it is obvious that some of the strips coming from that machine are much longer than 6 stamps, especially one of the first to be collected. (Thanks to Vince for this.)
This one on the other hand is a clear variety. Caused by an error in loading a roll into a back-office machine, the Cattle have the Diamond Jubilee overprint. This one was spotted on eBay.
Meanwhile away from Stampex, several offices had software faults producing Large Font errors (and of course no 10g denomination), including Camberley: