X, an accused drove off amongst his own lambs, without knowing it, a lamb belonging to Y. After X discovered the error, he sold the lamb belonging to Y with his own lambs. X is guilty of

X committed theft because he sold Y's lamb with full knowledge that it belonged to someone else. Even though X initially took the lamb by mistake, the mens rea (guilty mind) for theft developed when he discovered the error and dishonestly misappropriated Y's property by selling it. The initial taking was innocent, but the subsequent appropriation with knowledge of ownership constitutes theft under Section 378 of IPC.