f/k/a . . .

City of Chicago: Contesting a Parking Ticket – on this “I Object” webpage, Chicago lists each defense available for challenging a parking ticket, and supplies an email link for asking questions.  You need to check your local ordinances, of course, but this information would be valid in many communities:

1. 
   
   The respondent was not the owner or lessee of the cited vehicle at the time of the violation.
   
   
2. 
   
   The cited vehicle or its state registration plates were stolen at the time the violation was issued.
   
   
3. 
   
   The relevant signs prohibiting or restricting parking were missing or obscured.
   
   
4. 
   
   The parking meter was broken or malfunctioning through no fault of your own.
   
   
5. 
   
   The facts alleged on the ticket contain inconsistent or inaccurate information, or the facts fail to establish that the violation occurred.
   
   
6. 
   
   The illegal condition described in the compliance violation did not exist at the time the violation was issued.
   
   
7. 
   
   The compliance violation has been corrected prior to the hearing; provided, however, that this defense shall not apply to [various violations listed in particular sections of the City's regulations]  

And,

The National Parking Adjudication Service — It’s no surprise that a comprehensive site is available with parking-ticket information in the United Kingdom.  “The National Parking Adjudication Service is an independent tribunal where impartial lawyers consider appeals by motorists and vehicle owners whose vehicles have been issued with Penalty Charge Notices (or have been removed or clamped) by councils in England and Wales enforcing parking under the Road Traffic Act 1991.”  At the site, you’ll find information on Challenging Your Ticket, plus links to relevant Regulations and Legislation.