I experienced the same bug playing Jumpstart Historic Horizons on two occasions.
I had 1 vesperlark on the board and no creatures in the graveyard.
Opponent plays a spell targeting verperlark that gave it perpetual -1/-2, so vesperlark is sent to graveyard, triggering its ability. Because of the perpetual -1 power, verperlark's power in the graveyard is now 1, so it automatically targets itself.
Upon returning to the board, its toughness is -1 because of the perpetual -2 toughness caused by opponents spell. It is send back to the graveyard, triggering its ability again in a loop that can't be broken.
mobpsycho200#22729
I experienced the same bug playing Jumpstart Historic Horizons on two occasions.
I had 1 vesperlark on the board and no creatures in the graveyard.
Opponent plays a spell targeting verperlark that gave it perpetual -1/-2, so vesperlark is sent to graveyard, triggering its ability. Because of the perpetual -1 power, verperlark's power in the graveyard is now 1, so it automatically targets itself.
Upon returning to the board, its toughness is -1 because of the perpetual -2 toughness caused by opponents spell. It is send back to the graveyard, triggering its ability again in a loop that can't be broken.