Why Voltage & Reistance Testing can be unreliable

Everyone has done it, you are in a busy workshop and need to get the car sorted quick sharp.
So the consumer has power well it must be faulty, bang a new one on and....... oh poo.
Now I have to explain to the customer that I just spent £300 of his hard earned and the car still doesn't work.
In many cases testing the Voltage supply and the resistance of a circuit can really be a bad test.