It doesn't even have to be the ink running low or out. Very occasionally the press will fail to grab a sheet from the feeder. The next sheet pushes it forward and both sheets get fed in, the one on the bottom missing that color ink on all or part of it.
Or more commonly, there's a couple thousand sheets in the feeder, and the operator stops with a few to go. The unprinted sheets are used on the next skid of new sheets as a protector between the sheets and the wood of the skid. Later on, one gets picked up and cut along with the regular sheets.
Or..
The first few sheets through the press are what's called make ready sheets. They can be anything from nearly complete sheets to misprinted sheets to blank paper. There's usually only 1 or two of these to make sure the ink is printing evenly. Eventually the operator needs to use a few actual sheets to be sure of the registration. The last one of these is usually deliberately damaged usually by tearing off a corner. If they're not all used, the sheet sometimes gets into the printed pile, and eventually cut and packed.
Or ..
A piece of something usually paper gets into the press along with the sheet and gets printed, blocking ink to the sheet.
I've seen all of those happen, they all got caught during QC/handling at the shop I worked for. The first three are somewhat typical, the first means some maintainance on the press is needed. The last should cause a bit of trouble for the operator. Even if you're in a hurry and sort of skipping lunch by eating at the press there's no excuse for the bag that held your sub getting into the press. (If we printed cards that one would have gone home with me for sure)
Steve B
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