DISP=MOD does lead to a way to accidentally (or deliberately, as an exercise) create a "zero" BDW. With RECFM=F/FB/U and some LRECL, specified on the DD, write a lump of data with two bytes of binary zeros. Then with DISP=MOD again, and the "normal" RECFM/LRECL, write a good record. Then try to read the data. Tricky to do accidentally, but sometimes you find someone who errs, and then tries to "correct" without realising the full consequences.
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