Executive Meddling: The only way to describe how Steve Jobs muscled his way into the project over Jef Raskin's objections. Which might be a good thing, really, as the tension in the group already started to mount at the time.
Entire books could be and have been written about ideas on which Apple didn't follow through — the Taligent and Copland operating systems, the Jaguar platform (eventually replaced by the Power Mac), the Integrated Burrell Machine, the Macintosh Office, MacBasic, G5 PowerBooks, Cell Macs, and several possible mergers that never happened.
In 2001, Apple approached Sony because its Vaio notebook line was appealing enough that they were willing to allow Sony to make Mac clones. They even had OS X running on a Vaio laptop (note, this was before the official transition to Intel). Sony declined, on the grounds they didn't want to pull resources away for supporting two OSes when they were busy working on getting Windows optimized.
Mister Macintosh, an Easter Egg that would appear at random when the menus were invoked. It was halfway implemented then abandoned. It's legacy could be seen in the Kid Pix software, which drew icons in the menus.
The Mac was originally designed to use the Lisa's unusual 800K "Twiggy" floppy (its codename, from the 60s supermodel; the official name was "Apple FileWare" but no one except The Other Wiki ever calls it that anymore). Unfortunately, the Twiggy drive was unreliable and had worryingly low production yields, and in mid-1983, with mere months till the January 1984 ship date, the Mac team substituted the now-iconic Sony 3.5 inch floppy. It had half the storage in that first version, but it was pretty much ready to ship compared with Apple's in-house alternatives. Only two Twiggy Macs are now known to still exist; ironically, based on their use of a 3.5 inch disk rather than a Twiggy disk for the disk icon, they appear to run a version of the system software that came from after the decision was made to change drives, but before it actually changed. Their estimated worth as collectors items is well over US$100K each. (This was also an Oh, Crapmoment for the Mac team on multiple levels.) Apple wound up abandoning the Twiggy disk altogether, and the Lisa 2 shipped with a Sony drive as well.
Before acquiring NeXT and Steve Jobs, Apple considered basing the next version of Mac OS and Be OS and Windows NT(!)