An unforeseen (or perhaps foreseeable) issue; Most foot guns I have learned to avoid in Python are present in GPT4 generated code, and it is not immediately obvious anymore. Chased a surprising bug and found 
             C = [[{}] * m for _ in range(m)]
followed by 
         C[i][j][k] = "V"
buried deep. Because I trusted GPT4 (it aught to be good at these things) I failed to recognize this pattern :/