Yeah but there is no way "government mandated yellow dot pattern" is printed only on second side of the document... Must be something else. What happens if you run something thicker? I would imagine that running thin paper that has a little more moisture than it should would turn paper to be wavy and while it goes trough second pass it picks-up some toner dust deposits on paper path... Pretty much the only 2 things that touch second side of the paper are Transfer roller or belt and pressure roller. Look at them both - is there any toner deposits? I think big machine like this should have a transfer roller/belt cleaning device - is working correctly? Way to check is to turn it and see if surface being nicely cleaned. Also if machine has descent mileage there could be simple toner deposits that are dripping on paper occasionally... Make sure that machine is clean and NO Air can you will blow the dust from one spot to another possible creating bigger issues, 3M vacuum cleaner has to be used.