A. When the penis rubs against the toilet while taking a shit. This is mostly caused by a poorly designed lid.
B. When a turd drops forcefully through the water, scraping the bottom of the toilet where it leaves a streak.
What's wrong with your toilet man? I was hittin' porcelain while takin a shit.
Did you hear that loud Thoomp? Well that was me hittin' porcelain in your freshly cleaned toilet.