Oh god. So this happened to me but under very specific circumstances.
TTC for 10 years, 15 miscarriages, 12 or which were before 8 weeks so I'd been through a lot and due to totally irregular cycles I never knew when to expect a period.
Anyway, I had a 'period' then bled heavily 3 weeks later so assumed another miscarriage. Took a pregnancy test and there was a faint line so yep I thought - miscarriage again. We decided to stop trying because it was like the last straw. We then didn't have sex again.
Anyway, I had bleeds on and off on a roughly monthly schedules so assumed all was well. I'm a size 18 (this becomes important later). I thought I had a stomach bug about a month after the miscarriage because i was throwing up daily for a couple of weeks and felt really shite.
Anyway, 6 months after the 'miscarriage' (you can see where this is going) i suddenly got this walloping great 'roll over' feeling in my stomach that kept happening. It made me feel ill every time it happened (usually after eating) so I went to the doctors.
GP had a feel, immediately found my uterus and asked if i could be pregnant. I told her no because I'd not had sex since my last miscarriage, I'd had periods all the way through since then etc. Anyway, the midwife happened to be at the surgery that day so she grabbed her and her doppler and with a bit of wriggling about, we had a little heartbeat that wasnt mine 
I got sent up to the early pregnancy unit the next day for a scan and i will never forget the words in Feb:
'When did you say you miscarried?'
'July last year, why?'
'I don't think you actually miscarried. You're around 30 weeks pregnant and due in early April'
Lots of testing and scanning ensued, but all in all he was totally fine.
He arrived 3 weeks early, totally perfect and he's just turned 1 