The Teenage Mutant Ninja Turtles live in the sewer of Manhattan (in New York City). Sewers handle the bodily waste of several million people. There is no possible way that the Turtles could completely avoid coming in contact with human waste while moving through the sewer tunnels.
In many of the earlier episodes, a specific turtle's line would come out of the wrong character's mouth (i.e., Leonardo speaking with Raphael's voice).
The turtles are often referred to in the series as amphibians even though they are reptiles.