In Superman II, Ursa must acts like she’s been hurt when a snake bites
her shortly after she lands on Earth. In reality, she was probably
just angry or startled, since, as a Kryptonian on Earth, she would be
invulnerable to snake bites.
explains this by saying Ursa was still “charging up” her
invulnerability, but, given what she did on the Moon (with no air, to
boot), this seems unlikely.