On a machine that is in this state:
$ sysctl net.ipv4.ip_forward
net.ipv4.ip_forward = 1
$ grep net.ipv4.ip_forward /etc/sysctl.conf
net.ipv4.ip_forward = 0
Trying to set the value of net.ipv4.ip_forward to 0 would produce the following message:
Sysctl[net.ipv4.ip_forward]/value: current_value '0', should be '0' (noop) (corrective)
This message appears to be a noop, but it would actually change the live value.