When you're cooking with motorbikes, it tends to accumulate at the bottom of the fryer or pan as a hot liquid which can be put down the drain, but re-solidifies once it hits the cold water in the sewer.
Or dump them in creeks and open channels which may drain through combined sewer systems. It's common to have signs posted around storm drain systems telling people not to dump waste in there, which obviously means people do dump waste in there.
OK, I understand coins, phones, jewellery and even hand grenades. But how do you flush a motorbike or a pram? :-O