I sometimes receive E-mails telling me it is from the IRS and they found some anomalies on my last Tax Return. I would certainly find that unusual given that I'm Canadian and therefore the appropriate agency would be the CRA.
Though some of the scammers have adapted and do actually claim they are from the CRA. Notwithstanding that the CRA never actually contacts people by E-mail, they seem to just replace IRS with CRA. It will suggest there were "anomalies on your 1040". 1040 is the IRS form. the Canadian Tax Return Form is the T1 General.
Though, that is all intentional. Scams are less about "tricking" people directly as much as they are about funneling the most gullible people you can towards your schemes. If you can convince an elderly UK resident they didn't pay their UK income taxes to the IRS, you've hit scammer paydirt.