It depends on the speed and size of the centrifuge, the mass of the load, and the magnitude of the imbalance. Someone else mentioned an ultracentrifuge, typically a large, washing-machine-like device that can spin larger loads at high velocity. The amount of energy released if they become significantly unbalanced is pretty huge: they have a containment layer, but some could kill you if the load got through and hit you.

On the flip side, I may have intentionally ran unbalanced microcentrifuges a few (many, it was many) times as a grad student because I was too tired and lazy to make a counterweight. I just held it down with fairly firm pressure and it was fine. That's not very good for its bearings, though. Sorry lab manager!