That actually happened to my wife a couple of years back. We were walking round Mam Tor and Kinder Scout in the Peak District (UK) - for those that don't know, this is an area of boggy peat, so you sometimes need to be careful where you step. I'm pretty good at spotting what's solid and what's not, so I don't have a problem.
Anyway, we're walking along and I'm about 10 yard ahead, and then I hear this squeal and a shout of "Help!" from behind me. I turn round, and Emma had vanished up to her thighs in this mud-pit! Ever the gentleman, I pulled her out before I collapsed with hysterical laughter and took photos. To make it worse, she was wearing shorts at the time as well. It's a shame we don't have a photo of her while she was actually stuck, but it does mean we're still married... ;-)