Actually it could be a big problem, depending on her breed, what the bull is, etc. It can be an easy deal or a complete wreck. Potential problems.. calving difficulty - resulting in a c-section or cow and/or calf death. Mothering ability is less than a normal aged first calver. Milking - this is a biggie.. they are still in a heavy growth stage themselves so you have to feed the tar out of them to get even average milk production. Breed back takes a lot longer to get them bred back and stay that way.
We've had this happen before - more frequently that preferred so we have adopted a different management program accordingly. We routinely Lute all our heifer calves at weaning because we aren't sure. Occasionally one will slip by that and if we catch her in time, our vet pregs & if she isn't too far - Lute & Dex. And if they get past that.. we look at what she is potentially bred to and when exactly she could be due - according to our vet's estimation and what we think. If we decide to keep her, she goes into a place where she can be watched and then after she calves, she is fed like a show calf and we pull the calf really early.