yes this was my immediate thought after reading about handles with upper bits used for meta-data .... right before a shudder passed down my spine .... it brought back all those early bad mac memories
Early versions of Mac OS ran on the 68000 and 68010, which had a 24-bit address bus. The upper 8 bits of the address were completely ignored, so some parts of the Memory Manager (documented in my link above!) would use those bits to store flags for handles, like "locked", "purgeable", and "resource". Seeing that Apple used these bits, some third-party developers followed suit in their own applications (or accessed those bits directly as a questionable optimization).
When Apple released computers which used 68030 processors, which had a 32-bit address bus, this trick stopped working. There was an awkward period where "32-bit addressing" was an option which could be turned on and off in the Memory control panel for compatibility with older software (and hardware!) which wasn't yet "32-bit clean".
sure they used upper bits of addresses that the hardware of the day ignored, pretty predictably the very next hardware release (68020) supported 32-bit addresses potentially breaking just about every mac binary (initially they faked it out with a PAL in the address path, they eventually faked this out in the MMU resulting in a bit of TLB thrashing)
This was definitely a case of someone not planning to be successful and not looking far enough into the future
