On Tuesday September 30th, 2014, flight BA244 from Buenos Aires, Argentina to London, UK, was diverted to Sao Paulo shortly after departure, due to an unusual odor in the cabin. As the 777 was fully loaded with fuel for the long flight, it was unable to immediately land, as it was over-weight. So the plane had to dump fuel until it reached a safe landing weight. This is normal procedure, and perfectly legal. The fuel evaporates before reaching the ground, and breaks down in the atmosphere.
(Actual flight tracked down by @Trailblazer )
This fuel dump was captured both by Chris Bovey, a passenger in the plane, and by a plane-spotter on the ground in São Roque (about 40 miles from the airport). Bovey posted the video of the fuel dump on Facebook with little context, just the joking description:"I filmed this on a plane, what the fuck are they spraying from the airplane wing?" This was immediately seized upon as evidence of "chemtrails" - despite it looking exactly like every other 777 fuel dump video out there. Here's the result of a Google Image Search for 777 fuel dump:
[Update #1] When Bovey arrived back in the UK, he continued the joke, claiming he was detained in a holding cell for 8 hours (but somehow he still able to post to Facebook). This continued to fuel the conspiracy speculation.
[Update #2] Bovey appeared on the David Icke radio show "Volcania", and confessed that he simply posted the video as a joke, and that it was just a normal fuel dump, and that the "detained 8 hours" post was a hoax. He also stated that he did not believe in the "chemtrail" theory, which he felt was a distraction from real issues, and that this was all a good example of why people should check things before sharing them on social media.
Bovey's explanation/confession starts at 2:10
[This post is a summary of the information in the thread below, the original first post follows]