When I was first learning foundation single crochet (fsc), I found that I made the chain part of the stitch very tightly. This is because I wasn't pushing my hook through the loops all the way; I was just using the tip when I made the chain. When the chain part is tight, your fsc row will to curve in on itself, like so:
Three or four people have emailed saying, "What am I doing wrong?!?" and every time, they are doing the same thing I did.
To keep you from making the same mistake, I added a note to the foundation single crochet tutorial