But isn't the whole point of using layers that you can work in one layer for things like (in the case of forging a birth certificate) the entries you're altering/forging, but leave things like the basic form elements alone? So that if you make a mistake or are unhappy with the results of your forgery, you can go back and have another go at those parts
without altering the basic form elements? It doesn't make sense that a forger would go through the trouble of working with all of these different layers, only to place elements he's forging in the same layer as basic form elements that remain the same in any version of the document, forged or not.
Also, in the case of the "Name of Hospital or Institution (If not in hospital or institution, give street address)" line I cite above, both the form box in which that text appears, and the box above it are typewriter-entry boxes, with the typewriter entries coming nowhere near the "Name of Hospital or Institution . . ." text. Even if the forger had placed that text on the same layer as elements of the document that were being forged, it seems implausable that any mistake would have affected that text.
You CAN go back and start again, if you choose to do so. You can also flatten (i.e., merge the layers into one) the edited document before publishing it, but that wasn't done. But if you are non-competent or lazy, then you may well also allow the software to start new layers, as well as fail to flatten the final version.
Of course, it's possible that the forger was disgusted with what was going on and left the separate layers on purpose.