There are problems. The commonest are the so-called paradoxes. For example, if we could travel through time, imagine what would happen to a time traveller if he (or she) travelled back in time and killed his own grandmother at birth. In theory the time traveller will therefore never be born, so the journey could never have been made in the first place; but if the journey never occurred then the grandmother would be born which means the time traveller would have been born and could make the journey ... and so on and so on. This is a paradox.