How can you get pregnant with an IUD?

So this person found out she was pregnant while using a copper coil for contraception. I am a gynaecologist and I do see this fairly frequently, so how is this possible?
There are a few explanations; the first question I would ask is where is the coil? A copper coil is over 99% effective at preventing pregnancy only if it is situated correctly. When this woman went for her ultrasound scan her coil couldnt be seen. It may have been expelled (which is said to occur in up to 10% of users over 3 years) so the coil has literally fallen out at some point. Another option is it could have perforated through the wall of the uterus so it is sitting somewhere in her pelvis.
Either way, the coil isnt sitting in the uterus, so sperm can get around and fertilise as normal.
Another explanation is that there is a 1% failure rate, meaning that about 1 in 100 people will still conceive even if the coil is in the right place! If this does occur, the pregnancy is more likely to be an ectopic pregnancy. The coil doesnt actually increase the chances of an ectopic, but because it is so good at preventing pregnancies in the womb, any that do occur are more likely to be outside the womb.
So let me know - did this happen to you?