Two schema files at once
Two schema.prisma files in one repo meant edits landed in the one Prisma wasn't reading. Consolidated with an explicit path in prisma.config.ts.
Two copies drifted
One idea kept by hand in two places — schema and migration, dev and prod, app and app.
Both Backend/database/schema.prisma and Backend/prisma/schema.prisma were in the repo, which is how you end up editing one and wondering why nothing changed. Consolidated to Backend/database/ with prisma.config.ts pointing there explicitly. A non-default schema path needs to be written down somewhere obvious.