INES Mapper 100
Reportedly "MMC3/Nestice/Trainer/Buugy Mode Used in hacked roms !!"
Details are unknown. Sounds like homebrew hacks that work only on certain emulators, but not on real MMC3 hardware. Or it is just meant to be corrupted .NES files with garbage in reserved header entries at [07h..0Fh], thus changing mapper number 04h (MMC3) to 64h (100 decimal).
At some point, FCEUX lost support for this.
It's also currently used as a magic number by Nestopia-1.4.0 for "anything whose dumps only exist as UNIF (and therefore has no iNES mapper number assigned)"
Nintendulator uses this as a general-purpose debugging mapper, allowing PRG/CHR banks to be selected arbitrarily using a control dialog.