I remeber my first car, little Fiat Uno.... I didn't have the first clue about cars then, so I checked rad level and it was low, so topped it up with just water - probably had no coolant or antifreeze, whatever in it at all. Next time I checked it there was all this brown shit in it that actually did look like someone hand taken a crap in my header tank

Turned out to be rust from the inside of the rad gone all gloopy and looked like lots of poo