Birthinglau, good explanation.
I wanted to add this story. I am a chiropractor and did on internship with a chiro. who has been in practice for 25 years. He's seen a lot and told me this story about one of his patients. She was (and still is) a wellness patient and had been seeing him monthly for about five years when she got pregnant. She got through her pregnancy with the help of chiropractic care but had a hospital birth with an epidural. After the birth she did not see the chiro for a few months b/c she was so busy but returned b/c of persistent, severe back pain at the site of the epidural. He told her basically what birthinglau just explained to us, but when he did a physical exam, he decided to also take an x-ray based on his findings. You can imagine his surprise when a small, thin, sharp object showed up on the x-ray. B/c he took orthogonal views, he could see this object was between her vertebrae, in her spine! He referred her for surgery...the tip of the epidural needle had broken off and was lodged in her spine! Shocking? YES! And appalling that the anesthesiologist would not have said anything at the time--I think it would be pretty obvious that the tip of the needle was missing when he withdrew it! I do not think that's something you could NOT notice!
Obviously, this is a pretty rare complication, but it can and does happen. The scenario Birthinglau described is much more common.
ETA: I am not telling this story to scare anyone. I found it to be a clinically interesting story and an example of what *might* go wrong with an intervention, however rare. There are plenty of women who have epidurals during labor and have no back pain or headaches later.