The best unsanctioned public art story happened in Ottawa in the 1980/90s.
Lea Vivot felt that the plaza in front of (what is now) Library and Archives Canada on Wellington Street (next to the Supreme Court) "needed something". So in 1989, she showed up one evening with a truck and a crew and deposited a bronze sculpture of two young people on a bench. She "lent" the sculpture to the National Library, but didn't actually inform the National Library of this loan. The next morning, people arrived at the building to see new public art plopped down in front of the plaza.
In a glimpse into the workings of bureaucratic Ottawa, National Library/National Archives staff assumed that the National Capital Commission had arranged for the sculpture, NCC staff assumed that Public Works Canada had arranged for the sculpture, and Public Works staff assumed that the National Library/National Archives had arranged for it. Nobody bothered to confirm anything with anyone, and the public art sat there for almost a year with no one questioning it.
The bench turned into a popular spot for tourists to take photos. Increasingly tourists started asking staff at the front desk(s) in the National Library/National Archives for information about the bench, and National Library/National Archives staff started making inquiries within the government to find out the name of the artist, etc. That's when it came to light that nobody in the government had actually commissioned the thing, or knew anything about it. Soon thereafter, Vivot went public with her "donation". There was a fair degree of consternation among federal staff, as well as the artistic community in Ottawa, about artists unilaterally donating or lending art for prominent public spaces. I worked at the archives at the time as a student; staff there tended to think the bench was a bit too kitschy.
Not long thereafter, Vivot made another nighttime visit with a truck and crew, and she dumped a second bench on federal land to the east of National Library/National Archives on Wellington Street, this time closer to the Supreme Court (where, years later, the Tories would try to locate the awful Memorial to the Victims of Communism). This one featured a nursing mother, similar to Vivot's sculpture in front of Sick Kids in Toronto. This time the bureaucracy was quite annoyed, and in a huff, Vivot returned one night and removed both sculptures. Soon thereafter, she sold them both elsewhere.
The National Library/National Archives eventually made nice with Vivot, and commissioned some actually sanctioned public art from her. A new bench, very similar to the first, was installed in the plaza in 1994. It is still there, AFAIK. Vivot has sold a number of replicas since then.