The first fat binaries on the Mac were 68k-PowerPC.
That time one could also patch 68k OS calls with PPC code and vice versa. Depending on your viewpoint, making that possible, where the 68k OS calls had about 10 different calling conventions (the ABI of some of the callbacks seemed to be chosen by what registers the arguments happened to be in in the first implementation), that was either a can of worms or a brilliant hack.
That time one could also patch 68k OS calls with PPC code and vice versa. Depending on your viewpoint, making that possible, where the 68k OS calls had about 10 different calling conventions (the ABI of some of the callbacks seemed to be chosen by what registers the arguments happened to be in in the first implementation), that was either a can of worms or a brilliant hack.