Commodore PET
From Misc Data Digs
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If the user enters WAIT6502,x in a PET with Basic version 2, a string saying "MICROSOFT!" will replaced the text on the top left of the screen. x represents the number of times the string is printed.
It's also obfuscated in the ROM, hidden after a table of coefficients for the SIN function. Moreover, the initial revision of the PET ROM crashes if a user attempts to run the easter egg.