I ran into a very annoying case of this in the wild: a USB hard drive "pre-formatted for Windows and Mac". Specifically, the entire disk was pre-formatted as b

I ran into a very annoying case of this in the wild: a USB hard drive "pre-formatted for Windows and Mac". Specifically, the entire disk was pre-formatted as both NTFS and HFS+. The trick: as shipped, LBA 0 contained an MBR pointing to an NTFS-formatted partition spanning the free space of an empty HFS+ volume whose (slightly larger) partition was defined by an Apple Partition Map in LBA 1. So Windows would read the MBR and find the NTFS volume, Mac OS would read the APM and find the HFS+ volume, and all was right with the world. Right, that is, until you plugged the disk into a Mac after writing to it in Windows, because the simple act of auto-mounting the "empty" HFS+ volume caused OS X to fill enough of the volume's "free" space to corrupt the colocated NTFS filesystem beyond repair.