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This issue is fixed in the next update, 4.0.
When doing Skull damage with the Spiky Trait unlocked, Saguaro's Troop card art may remain over the board, on the other side or somewhere else it shouldn't be. This may also happen with Senita's Spiny Trait. This can happen on your team or the enemy's team.
Fixing the problem
To temporarily work-around this problem, Saguaro will return to normal when doing Skull damage again, including Doomskulls. Saguaro can still be used to cast Spells.