These simple mini kidneys — also known as kidney organoids — are grown from stem cells that are encouraged to develop into clusters of specific kidney cells. But it turns out that the “recipes” that encourage the development of specialized kidney cells were also cranking out cells from other organs, according to a new study.

That wasn’t what the scientists were expecting. Rather, they initially set out to grow kidney organoids in the lab and then analyze them to see what was happening inside of them, on a cellular level. To do that, the researchers looked at data collected from thousands of the organoids’ genes, representing more than 83,000 cells in 65 mini kidneys. They expected to see a diverse variety of kidney cells, comparable to what one would see in a normal, fully grown human kidney. But they discovered that 10 percent to 20 percent of the organoids’ cells were not kidney cells at all, but brain and muscle cells.