...except when the gate staff seee that you have deliberately lied.
Remember that the check-in staff are likely to be manning the gate later once checkin has closed. Some airlines make a point of noting the number of bags checked in and as hand baggage on your checkin record. We had a policy of charging excess for those that try and cheat the system. Either way, the gate staff have the same rules for items of hand baggage and will just take it off you there, where you then run the risk of it being left behind rather than re-opening the holds and delaying the flight if you try and turn up late at the gate.
I saw all the pax tricks in my time and grew weary of passengers who put their interests above those of the schedule of the flight at best, or safety of others at worst.