Dog pee.....right or wrong?

Yesterday, David and I took our pooch Casper with us to a very large outdoor flea market.  There were dozens of dogs there.  Casper was a very good dog.  He listened to us, never barked, and stayed by our side for the whole morning of walking around.

Occasionally, as dogs are want to do, Casper was sniffing and peeing on things other dogs had peed on.  We were very careful to make sure he didn't pee on anyone's products that were on the ground.  If you have ever had a dog who does this, they spray the equivalent of two or three tiny drops of pee on things.

As we were almost finished walking around the flea market after four hours, Casper apparently peed on one of the table legs of on a vendor's folding table.  I didn't notice because I only focused on him when we were near products on the ground or if there were other dogs around and I needed him to sit and focus on me.

I was looking at a lovely set of cufflinks at a table when this woman marched up to me and grabbed me by the shoulder to spin me around.  Casper, was laying under the vendor table where I was looking and was very calm.

"Your dog peed on my table."

"Oh, did he?  I'm so sorry."

"YOU didn't HEAR me....your dog PEEEEED on my table.  I have to put that back in my car."

"I heard you very clearly.  I'm sorry that my dog may have peed on your table leg."

"DINT you HEAR me?  Your dog peed on my table leg."

"I did year you and I have said I'm sorry."

"YOU are DISGUSTING FILTH.  That is disgusting.  You are gross and disgusting."

"Mam, I've apologized to you and am not sure what else you'd like me to do.  I'm going to walk away now."

"You vile piece of shit."

This lady was screaming at top volume with spittle landing on my face with hundreds of people stopping to listen.  I maintained my calm.  I honestly don't know what she expected me to do.

Ten minutes later, while Casper was giving a friendly greeting to another pup and we were chatting with a vendor, I felt a tap on my shoulder.

"You dropped this," said a man as he handed me a wad of paper towels the size of a softball. 

"Um...thank you, but I don't think I did."

"Yes, you did, you asshole, it's your dog's piss."

"Ok then."

Now the other vendor looked at me in an odd way.  I explained what happened.  She felt that it was very odd and told a similar story about her dog at a fair.  David was furious, as was his cousin.  I told them that we wouldn't stoop to the level of these people and I just needed to find a garbage to throw away the napkin.

So my question is this?  What would you have done?