“Hypothetical Situation”

This is a hypothetical situation as the subject already says...

If a speeding ticket is issued and for example states "speeding, 62 km/h in a zone of max 40 km/h" but one manages to argue that the speed limit is not posted thus speed limit is max 50 km/h, would this make the issued offence a fatal error?

i believe yes, because it would be the incorrect charge.