Source: Jyri J. Virkki
If you hold down + and press the Reset button, you get a digitized image of the three guys who wrote the echancements to the ROM's.
As an additional bonus, this trick served the purpose of performing a cold-boot of the computer, allowing you to break out of those annoying programs (mostly games) that required you to turn the machine off to get out of them.
In the built-in BASIC, you could specify CLS n (0<=n<=8) to clear the screen with various colors. If you specified numbers out of range, but <100, you would get an error message as expected, but the first time you did CLS n with n>=100, you would get a short message, again with the names of these individuals. Subsequent attempts would just give you the standard error message.