If I remember correctly, it's a pretty harmless feature: If Frodo wears the Ring too long, a neutral Eye of Sauron appears and sweeps the environment. Frodo then says something like "Soon the Eye will have found me", than after some time, the light focusses Frodo and he just takes of the Ring. IMO an extremly cool feature which is such a nice hint to the Parth Galen scene where exactly this happens.
Greetings
Melkor Bauglir