I don’t know, I’ve never seen one.
And yet, here’s the warning from MUNI / DPT / MTA / SFMTA:
“Warning: Avoid fake parking tickets
Internet criminals are using fake parking tickets in some cities to get people to go to Web sites that will infect their computers with viruses and other harmful software. If you get a parking ticket or notice in San Francisco, only go to Web addresses (URLs) starting with www.sfmta.com or www6.sfgov.org to resolve your ticket or notice. You can look up your ticket to verify it. Do not go to web addresses with an @ (at) sign in them, for example, email@example.com; they are usually harmful.
Provided you have PDF-reading software loaded on your computer, the San Francisco Municipal Transportation Agency (SFMTA) will never ask you to download software in order to validate your parking ticket. Be very suspicious of Web sites that ask you to download software in order to validate or process your parking ticket.”