What are common complications of a facet joint injection?

Pain, numbness. Facet joint injections are often done in the lumbar & cervical spine, often under fluoroscopy (live x-ray), bathing this joint with an anesthetic (lidocaine, etc.) and steroid/cortisone. Common side effects include pain (during and for the first 48 hours), numbness (often temporary) and bleeding (usually minimal). Rare & serious side effects include infection, nerve injury and spinal fluid leak.
Risks. There are risks with any procedure including bleeding, infection, nerve damage and paralysis. These are rare and very often never seen in properly trained physicians such as fellowship trained pain specialists. Also make sure they use fluoroscopic x-ray guidance to verify location.
Usually minimal. Including some increased pain at the site and rarely infection, nerve irritation or nerve damage or scarring. Rare too, any spinal fluid leak. Occasionally, an allergic reaction to one of the substances injected.