If you mean that the laminations are all missing the I piece,that's a problem,but if it's only the last lamination as in the end of the transformer,probably doesn't matter too much because it doesn't need it.
If the laminations are loose,the core will wobble and that's a problem (It will also wobble when it's firm and set,but it will do it ever so slightly. Serious wobbling will lead to losses and damage to the transformer.)
Maybe a picture?