The plumber would be required to give a receipt to the buyer - and the buyer should insist on one just in case. If the plumber refuses the request for the receipt then he is both violating the law and almost insisting that he be reported.
Come down off the drugs, you're rambling.
The bill is the record of the transaction Your Denseness.
You're making the assumption the mistake was somehow dishonest. Your hate and distrust of business people (or maybe just plumbers) capable of being honest is showing as well.
Not at all, looey ... and YOU'RE making the assumptions that both parties "forgot"; the plumber after he had agreed by contract to collect such taxes.
Read the bill's requirements that the receipt that must be provided shows the tax-inclusive taxes. The "... shucks, I forgot" defense won't work too well.
You're making a massive stretch (also unwarranted) to try to throw insults about.