Comment by bitwize

The Level I BASIC for the TRS-80 (which only shipped with 4 KiB of memory originally) had three error messages: WHAT? (syntax errors and the like); HOW? (illegal operations like divide by zero); and SORRY (out of memory).

BootOS, the 512-byte OS written by Oscar Toledo (author of this article), also has a single error message, "Oops".