First of all, I envy those of you that have only stepped in pet excretions twice. One of my cats used to have IBS and a hairball issue - I probably stepped in some variety of slime on a weekly basis, and on top of that, she'd frequently get it on the back of the couch too, so I also would end up putting my hand down in it.
Second, I think the most disgusting thing that I've ever stepped on was a lizard. I was getting out of the car and it darted right under my bootheel as it was coming down. I rather like lizards, so I was especially disturbed by accidentally killing it in a very gory fashion.