I had a copper coil inserted after my DD was born (she's now just turned 17). I was told it would need replacing after 5 years or so. When that time came around I went in for a consultation and they said they couldn't find the strings. Further checks were done and they said I'd need a D&C and they'd check to see if it had been expelled. I thought this was weird, surely I'd have noticed a coil coming out but I did have horrendously heavy periods at the time.
I knew by that time that I was definitely done with having children so asked if they could tie my tubes at the same time. Doctors agreed and so I had the op.
All went well. A few days later I received a letter saying that they'd given me a D&C, tied my tubes and that they hadn't found any IUD during the operation. I just assumed that somehow it had been expelled and I'd not noticed. I've not really thought about it since.
This week I had some x-rays done at my chiropractor for a hip problem I've been having. Guess what shows up on the x-ray? An IUD!!! My chiropractor commented as it's now upside down but she thinks it is still in my uterus 
I'm going to go to my GP next week and see what they advise about getting it taken out now.
I'm pretty annoyed though, how could they have missed it when I was treated last time?? My periods have always been heavy - tailing off a bit now as I'm hitting the peri-menopause - but have I been suffering unnecessarily for the last 12 years due to the copper? I'm always anaemic too due to the heavy periods. If I don't take iron tablets every day I get quite ill.
I've done a quick google and there's nothing about any long-term issues from the coil being in for a long time. Still quite annoyed though, especially if this is the cause of my ongoing period problems/anaemia. Has anyone else ever experienced similar?
Sorry it's so long!