There was also the MFS/HFS thing. I know some software would not run under HFS, but could be remedied by running an MFS system, and/or keeping all files at root level. However there were a few apps that would not run even under that configuration on a 128K ROM. Fortran comes to mind. Dollars and Sense, Q&D Filer, 1st Base, MacDraft, MacDraw, Guided Tour, and MacServe also had incompatibilities. I've also read that a number of games required Fedit to change the screen address in order to work properly on a Mac Plus. The bigger problem I have personally run into is software that did not know how to deal with more than 128K or 512K RAM. But that's another issue altogether.