(But way more efficient methods certainly exist as the GBA emulator [https://youtu.be/Sfc_1NKbiKg SkyEmu demonstrated super long rewind] while needing very few RAM)
For debugging, going back to the previous instruction is easily implemented. The emulator stores has to store the time when the previous instruction was executed so that he can go back to that instruction. In the same way, emulators can provide access to the previous HBL, IRQ, VBL, etc.
= Emulator accuracy checking tools =