People pulling trailers down the road that have no working lights on it!
Sometimes you might even see why the lights don't work ... Pigtail not plugged in, or the lights are missing all together!
Don't people walk around there trailer to see theses things, or glance in the mirrors every few hours???
And speaking of the occasional trailer pullers, especially the 8'+ wide ones, most seem to forget that they are wider than the tow vehicle, who drive right on the center line, with trailer out of there lane!!!