Are expired injectable fillers harmful?

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 But it is a ethical question of using out of date fillers.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If you had a reaction from out of date fillers then the doctor has preformed malpractice and has license issues.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 fillers. I guess to save a few dollars.
She stated she had Zyderm injected but was not sure. On examination there was an ulcerated nodule of the lateral upper lip less than 2 cm. No exudate was seen. I started her on PO antibiotics and steroid low dose injections. The eruption and ulceration cleared in 2 weeks but the nodule only decreased by 50% after 3 injections of Kenalog 10%, 0.1 cc's. I had her request her records from this clinic. It took her 3 months to obtain a one page office note.
During that 3 month interval, she had another episode of swelling in the area which I treated. Upon review of this record, it stated she indeed was treated with Zyderm. A label was a fixed to the injection date. The next observations were very insightful.
The date of expiration was 8 months earlier, well after the injection date. And NO TEST DOSE WAS DONE! So 2 areas of falling below the standard of care. But it gets better! I did a biopsy the nodule with a pathology report in a few days stating Silicone granuloma. Unfortunately this clinic had no insurance so there was really no recourse for this patient.
The point is be careful out there.

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.