Hi johnnychips -
Well I've never had to give the "wrong" answer via online check-in, but I have at the airport. A couple of years ago I checked in at MAN for my Jet2 flight to Pisa and was asked, "Are you carrying anything for anybody else?"
"Yes , I am" I replied, as indeed I was. A bag full of home comforts for a friend based in Pisa for a year to study Italian at the university there.
The poor trainee check-in girl looked stunned and scarcely knew how to reply, but her supervisor soon stepped in and directed me to have my luggage x-rayed nearby prior to acceptance for the flight. And that is all that really needs to happen; an extra layer of security checking for the affected luggage.
Hope this answers the question.
SHED.