I have a frequently spammed script on one of my sites. I was suprised while reading my server logs to discover that one of the miscreants showed up using...

Mozilla/4.0 (compatible; MSIE 6.0; MSIE 5.5; Windows NT 5.0) Opera 7.02 Bork-edition [en]

Certainly it is a spoof, but an unusual one. Of course the Bork-Bork version of Opera was a retaliation against Microsoft after none of the MS engineers were able to produce a webpage viewable in the Opera browser. This version caused Microsoft pages to appearas if they had been written by the Muppets' Swedish Chef.