OMG I had that happen to me once. There was a trail I regularly rode on and there was a very small wooden bridge - maybe 5 or 6 ft across, and it was old and rotten so I was afraid to ride on it. I guess I probably had not ridden this trail for a few years for the bridge to have deteriorated enough for me to be afraid to go over it. I went around this time because the ground looked frozen and my horse sunk in black mud up to his belly.
I wish it was in the days when we had cell phones and cameras in our pockets so I could have taken a picture. And it sure would have been nice to have a cell phone to call for help as well because I was by myself and felt that I could not leave him there alone because if he got out he would run home and that involved crossing a highway. It was a big conundrum.
He was such a good horse. He did not panic at all, he just stood there, legs completely trapped, looking at me like 'what now mom?'. I got him out with a switch, it wasn't pretty - let's just say he needed some serious encouragement to haul himself out of there but we were lucky because solidly frozen ground was only a step away so once he climbed out he was safe. And I was lucky that he didn't pull anything hauling himself out of there. I checked him out, climbed on and rode home and he was no worse for the wear. Phew!
Although it has been an incredible season in terms of riding-friendly temps, the down side is that the ground is not frozen the way it normally would be around here. This incident is a perfect example of what to expect out there. Anywhere there is surface water/swampy conditions the rest of the year, it's still boggy under whatever ground cover there is. Be careful!!