If the targeted Shapeshifter becomes an illegal target but the targeted creature doesn’t, the ability will resolve but have no effect.

If the targeted creature becomes an illegal target but the targeted Shapeshifter doesn’t, the Shapeshifter will still become a copy of the object. The effect will use the illegal target’s current copiable characteristics if it’s still on the battlefield, or its last known copiable characteristics if it has left the battlefield.

The copy effect doesn’t wear off until just before your next untap step (even if an effect will cause that untap step to be skipped).

If Shapesharer itself becomes a copy of another creature, it loses both changeling and its activated ability (unless it’s copying another creature with changeling and/or another Shapesharer, of course).

This ability can cause a creature to become a copy of itself. This will usually have no visible effect.

The targeted Shapeshifter copies the printed values of the targeted creature, plus any copy effects that have been applied to it. It won’t copy counters on that creature. It won’t copy effects that have changed the creature’s power, toughness, types, color, and so on.

If the targeted Shapeshifter copies a creature that’s copying a creature, it will become whatever the chosen creature is copying.

If the targeted Shapeshifter becomes a copy of a face-down creature, it will become a 2/2 creature with no name, creature type, abilities, mana cost, or color. It will not become face down and thus can’t be turned face up.

Effects that have already applied to the targeted Shapeshifter will continue to apply to it. For example, if Giant Growth had given it +3/+3 earlier in the turn, then this ability makes it a copy of Grizzly Bears, it will be a 5/5 Grizzly Bears.