Zamasu (Dragon Ball) had his immortality compromised by fusing with Goku Black, a mortal being, thus leaving him more vulnerable.
Bender Bending Rodríguez (Futurama) is a robot built without a backup unit, so he does not possess the immortality that he boasts about, rendering him a mere "mortal coil".
Utsuro (Gintama) was on the verge of being mortal when Umibozu jammed an Altana of a foreign planet into his body, and even recreating a new body from his severed arm has greatly depleted his supply, if he were not to return to Earth to recharge.
Kouka (Gintama) became mortal once she left Kouan, ceasing to draw from its Altana, and when the supply she accumulated ran dry, she died.
When Castiel (Supernatural) lost his grace to Metatron, he became human and mortal
Metatron (Supernatural) became mortal when Castiel removed his angelic grace
After the destruction of the branch of sin. Shiro (Deadman Wonderland) lost her nigh-meta regeneration healing factor, leaving her weakened to the point that she requires constant medical care in order to survive.