If you tune a piano, say, by starting at, say, C and tuning pure fifths around the circle of fifths, when you get back to C you will find that the distance from the F to the C is a little flat of a pure fifth. This discrepancy is called the Pythagorean comma. More details for the interested student are here:

http://www.medieval.org/emfaq/harmony/pyth.html

Note especially the section 4.4. The two commas: bugs or features?