A few weeks ago, we had a new garden fence put up which needed sprayed with wood stain/preserver. Went to B&Q and saw the pump action spray kits they have which supposedly do the job just as well as a brush in a fraction of the time. Sprayed the fence (on a clear, calm day), ended up redoing most of it with a brush, decided that the B&Q sprayer was a total waste of money and thought nothing more of it until a couple of weeks later when I was washing the Golf.
As I washed the wing nearest where the fence had been sprayed I noticed that the paint was now covered with tiny spots of brown, as was the whole car (even the far side, although not as badly), and it eventually dawned on me that this must have been caused by fine spray drift from doing the fence

even though the Golf was several feet behind me at the time, there was virtually no wind, and I was spraying away from it.
After the obligatory 10 minutes swearing at myself for being such a moron, I got the T Cut out and checked that the bloody stuff could be removed (car shampoo had no effect). This worked but there was no way I was T Cutting the whole car so it was off to the local valet place (The Washbay in Aberdeen) to get it sorted out. They did an excellent job and told me that this type of cock up happened surprisingly often and was a great source of income for them!
Cost me a lot of money but I've certainly learnt my lesson.