PDP-20
The most famous computer that never was. PDP-10 computers running the TOPS-10 operating system were labelled "DECsystem-10" as a way of differentiating them from the PDP-11. Later on, those systems running TOPS-20 were labelled "DECSYSTEM-20" (the block capitals being the result of a lawsuit brought against DEC by Singer, which once made a computer called "system-10"), but contrary to popular lore there was never a "PDP-20"; the only difference between a 10 and a 20 was the operating system and the colour of the paint. Most (but not all) machines sold to run TOPS-10 were painted "Basil Blue", whereas most TOPS-20 machines were painted "Chinese Red" (often mistakenly called orange).