Do you mean unchecked, as you could also access information that is "out of bounds", so, it would just return "rubbish", I guess?
Unchecked as in it will assume you know what you're doing and let you shoot yourself in the foot. The usual term used to describe behavior for using out of bound indices is "undefined behavior."
If you're using VC++, even  is checked in debug mode - using an invalid index will trigger a debug assertion. If I disable that and use the following code, with unchecked_out_of_bounds_access uncommented, the program will crash.