The Star Wars robot-naming convention. It’s an entire galaxy which has massive consumer take-up of personal, household robotics. Yet robots are uniquely named by a four-character alpha-numeric code?
Apparently the actual genesis of R2-D2’s name came from the production American Graffiti. The appellation was shorthand for “Reel 2, Dialog 2,” a designation within the sound editing process. While Lucas was working on the Star Wars script and sitting in on the sound editing for American Graffiti, overheard sound editor Walter Murch voice-slate* a mix as R2-D2. He immediately decided it was a great name for a character.
Which, mathematical issues aside, it actually is.
*A spoken marker on the sound recording that is the equivalent (and matches) the video of the “clapperboard.”