I could not find an answer for this in the Wrenchmen's Toolboxes, nor from searching the forums.
Randomly, when playing, a trail from the back of a Champion or minion will appear and stretch back to the Nexus. At random points, the trails will go away, but come back on different champions or minions. See attached SS for visual explanation.
I have tried changing the shadow settings, which oddly only changed how the trails had shadows. Fast Terrain did not change anything, except turn my shadows off completely, but the trails still existed (without shadows). I tried changing every other setting from lowest to highest, but always got the same problem.
Reading about the black texture problem, I made sure my card was on application controlled AA/AF and the default were both off.
I tried updating my graphic drivers also, but the problem persisted even after that (with a complete driver clean beforehand). All my other games seem to run fine, so I don't think the video card is going.
Edit: After installing new drivers and getting the same problem, I Repaired, then tried changing all the settings again, but still got the trail randomly.