[Bytsi]

You mean curl (not arch), right? As in your lower back is going the opposite of its natural curve (kinda hunching over instead of being nice and upright)?

Any time you hunch over instead of keeping that "good posture" back while lifting heavily, you risk injury. I would say that any time you can't maintain good SAFE form on a lift, you've hit failure and need to stop that set.

Hope that helps!!!