If I am walking and get one foot on solid ground, and the other in the mud, I shift over to get both feet on solid ground. It's pretty much automatic. I can't imagine anybody intentionally walking with one foot in the mud and the other on solid ground. Unless, of course, that person was intentionally trying to confuse anybody who saw his footprints, as a joke... ;-)
Then i must be weird, cause i have done it a few times just for fun. Comparing how the mud and solid ground affect both feet different is interesting to me, call it scientific playing.