Should programs be able to output to STDOUT, when the question says you must "error"?
For example, the python program
ZeroDivisionError: integer division or modulo by zero. This is to STDERR by default.
But the program:
print "ZeroDivisionError: integer division or modulo by zero"
Echos the same thing, but to STDOUT. There is no observable difference in the output of the first and the last program, save the STDOUT/STDERR issue.
In addition, we already allow output to be piped to STDERR as a standard i/o method.
Some programming languages, such as Mathematica, even output errors to STDOUT by default.
So, should we allow answers to output to STDOUT when an "error" is requested?
And the opposite should also be considered: what about answers that output errors to STDOUT, which would normally fall under the "No Extraneous Output" rule?
Related: Programs may output to STDERR