Yep. We have a mare who checked not in foal at the end of the 2004 breeding season; it was late so we figured we'd start early the following year, so we put her under lights that Winter. She is an old mare who's had a lot of foals, so she always has that swaybacked, full bellied look whether she's pregnant or not, so in her case, appearance wasn't helpful. Yeah, she looked pregnant, but she ALWAYS looks pregnant! And, she's been known to lactate when she's not in foal, too... so even her udder is not a good indication of her status.
We tried and tried, but she would not tease in. We actually thought maybe her age was catching up to her and she wasn't going to cycle. We turned her out with our stallion for about a week or so. She was NOT in heat during this time; we had her in there as a companion mare for him. One of our other mares was confirmed in foal and put in with him, and she came back out. Still, she wouldn't tease in.
When the vet came out to palp another mare, I asked her to palp this one too, let me know if she was cycling at all, etc. She got this funny look on her face and asked if the mare had been exposed to our stallion. I said yes, a couple of weeks before, but I was sure she wasn't in heat at that time. She asked if she'd been exposed to him before that, like last year? I just looked at her for a minute trying to comprehend what she was getting at, and she informed me that she was "petting a foal... not a fetus; a FOAL."
So, a short time later she foaled and we named that filly (out of Indecent Proposal and by Indy Mood) "Inysent Surprise."
Edited to add: I think what happened is that this mare has a cyst that is about the size of an 18 day pregnancy... I think the vet found what he THOUGHT was that cyst, only in this case it was really a pregnancy. Or the pregnancy was hiding behind the cyst, or something...
