But the real surprise came two minutes later when police arrested one dented protester for causing criminal damage to the car's bodywork.
If that incident seems bizarre, consider the case of Lawrence O'Dowd, an unemployed 18-year-old, who was arrested earlier this year for saying "Miaow" to a police dog. The charge: using threatening words and behaviour.
Since 1995, the Brighton based underground newsletter SchNews has collated a "crap arrests of the week" file containing hundreds of incidents. While the reports have a humorous side, they indicate that police sometimes make vexatious and frivolous arrests.
When in doubt, police make use of conditional charges involving suspicion. Three Brighton protesters who last year occupied virtually indestructable steel CCTV camera poles were arrested for "suspected criminal damage", while a a Derry man, Liam Forbes, found himself under arrest for "behaviour likely to lead to a breach of the peace" after taking his coat off to reveal his team colours. All charges were thrown out by magistrates.
Arrests involving suspicion are particularly popular when dealing with demonstrators and activists who can then be held in custody for up to 24 hours, or barred from protest sites.
Take the case of the Newbury by-pass protesters, who were pulled over by Thames Valley police while on their way to a demonstration. As they waited for some minutes on the roadside, one of the men unbuckled his seatbelt, and was promptly arrested - for not wearing a seatbelt.
One Sussex man, protesting outside the home of a Kent vet who signs live animal export certificates, was arrested for "causing alarm, distress and harassment" after he called a policeman by his first name. "George" then imposed bail conditions not to go within a 20-mile radius of Dover.
Later that month, during an Anti-Live Export action at Woodsfarm in Redditch, a woman was arrested for being in possession of a pair of nailclippers. She was later released with the warning that if she took them on another action she would be charged with "going equipped".
A London Animal Rights' activist protesting outside a gun shop was arrested for possession of an offensive weapon - a bicycle lock. He was eventually released without charge.
Arrest can lead to more serious consequences. A man reliant on self-dialysis equipment was on his way to a demonstration at Hillgrove Farm, where cats are bred for vivisection. He was stopped by police, who then searched his car, discovered the medical kit, and arrested him on suspicion of using equipment to rescue the cats.
According to the SchNews report, the police claimed the boxes in his car were "cat boxes" and would be used to take liberated cats from the farm. The police wouldn't believe the boxes contained self-dialysis equipment which the man needs to use four times a day, even though "dialysis" was written all over them.
Before being taken to Oxford police station, the man was kept in a police van for two and a half hours. He was released 30 minutes before the deadline for his next dialysis, but it then took him another two and a half hours to get back to his car.
Two Christchurch men leaving a pub were asked by a passing policeman to reveal where they lived. When one replied "at home" the two were arrested and locked up for the night.