HOMMII. Heroes of Might and Magic II runs on 8.6 but will not work on properly on System 9.x. The game will launch, and you can play, but if you try to save or load, it will crash.
This is a pain because my son (11YO) and I both like this game. I have a frankenized Umax S900 with 1GHz G3, but he has an 800 MHz iLamp and the iLamp won't run anything older than 9.2.2, so there's no way to play a networked game of HOMMII together. My old S900, of course, has boot volumes for 7.6.1, 8.6, and 9.1.
So, it's only one example, but there's one. Of course, HOMMII will run fine in 7.6.1 as well, so there's still not a lot of impetus to have 8.6, except.... I had a hard drive crash a while back, and I can't restore my 7.6.1 volume because I have a 7.6 installer and a 7.6.1 updater. So I have to run HOMMII while booted from my 8.6 volume until I can recreate the 7.6.1 volume.
When I try to instal 7.6 the installer tells me it won't run on this machine, because I soldered PM9600 Enhanced (Kansas) ROMs onto the S900 board. So it thinks it's a Kansas machine and wants 7.6.1 or better. I guess there must have been a direct 7.6.1 installer out there somewhere but I've never seen it. I can get around it by creating an installation on an older machine, but then I'd have to set up an older machine. Time....