people trying to flog something on your doorstep. You politely tell them no thanks ... and they carry on??? I just give them silence from then on .. then it becomes the awkward eye contact thing ;0)
Dog crap hanging in plastic bag from bushes at the side of the path.
From the lazy prats who know that they have to pick it up in case anybody is watching but as soon as they know that nobody is about they throw it away.