An introduction to Pluto

In a Jupyter notebook, make a cell containing "a = 0" and run that cell.
Then make a cell containing "print(a)", and run it. It will output "0";
Make a third cell containing "a = 1" and run it.
Now run the second cell again. It will now output "1".

You now have a notebook looking like this:

a = 0

print(a)
-> 1

a = 1

You can now delete the third cell, making it even more confusing. In this example it's obvious that a might have an unexpected value, but in real notebooks the value of a often just used in calculations, not printed, making it unclear on exactly which values the calculation is based.

The problem disappears of you always run all cells from top to bottom, which is not hard to do, but Jupyter makes it even more easy to run selected cells one by one as users see fit. It's one of the main criticisms of Jupyter. I haven't read the article, but it looks like the Pluto authors have managed to avoid the problem altogether in Pluto.