I received this error after killing the baby buffalo on day 3:
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FATAL ERROR in
action number 1
of Step Event0
for object oenemy_bull_big:
Unable to find any instance for object index '51' name 'oenemy_bull_small'
at gml_Object_oenemy_bull_big_Step_0
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stack frame is
gml_Object_oenemy_bull_big_Step_0 (line -1)