My favorite error message was produced by the Univac Fortran V compiler, circa 1970: “Warning: floating point equality tests are nugatory.” I pride myself on my vocabulary, but I had to use the dictionary.
I recall from 1965 getting an error message message something like this from the Fortran complier on a Univac 1107 system after receiving too many error messages: "Do not attempt to learn Fortran using Monte Carlo method. Buy a manual in the user office."