Sans is a Sans variant created by LoverofPiggies (aka CrayonQueen). Originally a version of Geno of After Genocide, he fell into a dimension between Universes and became glitched beyond repair. He now dedicates his life to cleanse the Multiverse of Anomalies, even if it means destroying all of the AUs and Out! 
Does error Sans remember his past?
Error may have been Geno, but he doesn't remember that. He doesn't remember being the bloody skeleton, or any of the things his previous self did. 