Tends to happen after not shitting for a while, eating a big meal, when you are sick, or when you've eaten some crappy food.
Stage 1: Normal shit: Comes out looking normal, but all at once.
Stage 2: Half-solid shit: Comes out all at once and is only half-solid.
Stage 3: All liquid: The shit just pours out of your ass in a shit-smoothie. Ass-washing is required.