I didn't introduce this, it was already there in tapdisk. I don't see a problem with it as the start of the L1 table is referenced in the normal qcow header. qemu-img sets this to something like 0x48 which is immediately after the header, tapdisk uses 0x1000 and gains some unused space for things like the extended header. This is compatible with the qcow implementation of qemu/ioemu.

On the other hand, I could simply strip that extended header (i.e. overwrite the magic with 0x0) after having fixed the image. Then it wouldn't be detected as broken on the next start as well.