I worked a couple of special event stations on 20m PSK31. DL100OUI was a special call to commemorate the 100th anniversary of the first commercial radio communication between Germany and USA. The operator was Reinhard, DK1IO.
The other one was S503EO which was to commemorate 50 years in amateur radio by the operator Milos, S53EO!
That gives me an idea. My 60th birthday is in a few weeks time, perhaps Ofcom would let me have a special event call to mark that? :)