a 'borrowed neutral' is generally found in domestic lighting circuits especially the landing light. It will use the live of one circuit and the neutral of another, as as been stated, harmless to the user of the property but a hazard to sparks and wannabe sparks as someone changing a fitting further upstream on the circuit which the neutral has been borrowed from could get a belt even though the circuit appears to be off if they break the neutral.
IMO C2 any day of the week, it is a contravention of the regulations and always has been. But virtually no danger to the householder
Far more dangerous IMO is the shared neutral, 3 lighting circuits accross 3 pahses sharing 1 neutral 3 seperate mcb's. I have witnessed a sparks get thrown off his steps when he got caught out with that one!