The following story serves as an example that you should always pay attention to what you are knitting.
I went to a knit meet on Wednesday and knitted happily along for several hours on my Feb Lady Sweater without noticing anything odd, and I actually got quite a lot done. The next day I started looking at what I had gotten done, and looked at the progress. And noticed that part of the knitted fabric looked somewhat strange. You can't really tell, but here's a picture
The knitting looks looser and more "holey", and I realized that part of the yarn was significantly thinner than the rest. Well crap, now what should I do? I thought about leaving it the way it was and pretend that it didn't bother me, but I know that it would have bothered me, so I decided to rip back to where the yarn went thinner, which meant that I had to frog almost all of what I had knitted the evening before. :( Oh well..
Here's a pic to show you that the difference in thickness was really significant, it suddenly went from 6ply to 4ply
I still can't get over the fact that I didn't notice this while knitting it! Anyway, I've been in contact with the shop that I bought the yarn from, and they promised to send me some more yarn in the same color, to make up for the change in thickness. So I'm not too bummed about this whole thing. :)