If you look down into the machine with the top door open while loading, you can see the paper feed ahead of the long black presser bar. It has to "park" ahead of it about 3/8" evenly on both sides before the head starts printing or you'll get a skew error and it will not accept it. Same goes for canvas feeding which can be a nightmare.
Fwiw, that rear pull-up tray has a bit of a slant on a couple of my Epson printers which exacerbate the skew error. Found it on longer 36" cut pieces where it prints a bit sideways on the borders. I saw on dpreview where someone made a tube wrapped in tape that he put on the flip-down guide on the right side that cants the paper a bit to the left, and then a spring clothespin on the left side as a guide rail for the left side. Seems to work for me too.
I have used both hands on the paper surface to flatten and get it to feed straight at times as any lengthwise curl interferes with the 3880 feed. With canvas, I have tugged "hard" on one side while initially feeding to get the thing to look even ahead of the black bar so it would throw a skew error and not feed it. With practice it does get easier, but watch to make sure the paper is parallel and even with the black edge of that bar or else. I did the taped leader once but it came loose and I had to take it apart to get it all off the feed system so I wouldn't go there, plus you have to account for the print to be moved down the page a bit too with the leader method.
SG