Although conceptually, it is not the best idea to be injected with an expired product, the chances of any problems are minimal. Studies have been conducted on hyaluronic acid gel showing no bacterial, or fungal growth for 2 years when stored at room temperature.
Most likely they are not harmful. But it is a ethical question of using out of date fillers. The old 'what if' issue comes into play.
If you had a reaction from out of date fillers then the doctor has preformed malpractice and has license issues. An example in a sense that I was involved in here in Miami, I saw a patient 3 or 4 years ago who had an eruption nodule on her lateral upper lip for over three months. She had implants from me years earlier but decided to use a Latin Clinic for her...
I agree with Dr. Placik that it is very unlikely that you would have a problem. Oftentimes, medications and injectables have dates because the FDA requires them. While it certainly it is not advisable to use fillers beyond this date, since sterility cannot be ensured, if the filler was not opened or disturbed it should be OK.
I tis highly unlikely that this will harm you. Having said that, I would not make that a routine nor would that be the standard of care. Essentially these dates are intended to be adhered to by practitioners.