This is especially true to the people building the ROM emulators. There are so many tricks/hacks/kludges that were at the heart of some games even being usable. Timing for interlacing that was needed for the game to work in NTSC has to be accounted for on today's faster hardware and progressive scanning. Reproducing color accurately from NTSC seems to also be another thing I've seen. I'm sure there are plenty more that have been posted here before, but they are always a fun reminder that porting code can be a nightmare.