I used a free paper piecing pattern of a snowflake design which I found on the web. It was my first attempt at paper piecing. The pattern produced mirror images of what it was supposed to! I thought it was my fault after I tried twice and got the same result. So I brought the pattern to a friend's house (who had done pp'ing before) and had her give it a try. Same result - mirror image.
I probably should contact the author. What I don't understand is she has comments on her web page written by people who used the pattern and were successful with it.