My older cat just did this to me a few days ago! He supervised while I cleaned his box (right on top of me)... I poured in some new litter and just as I was about to spread it out he could wait no longer, he shoved me out of the way and stepped into the box... I was like "No buddy wait, I haven't put the top on and Im not finished!" Too late! He stepped in, looked at me (gave me that what are you lookin at face) and took a huge poo... I was laughing so hard (he looked like he could have used a newspaper by how serious he was going about his business)! The strange thing is that he actually covered his poop up for me (he never does this), I guess it was his way of saying thanks mom, sorry I couldn't wait any longer