Why not modify forward hook output?

In the PyTorch documentation under register_forward_hook, it says “The hook should not modify the input or output.” (https://pytorch.org/docs/master/nn.html#torch.nn.Module.register_forward_hook)

What would be the danger of modifying the output? This could be done in-place, and I was wondering what would be the consequences of doing this? (i.e. what would break?)