I just received my sleep sack that I was planning to use for my 3-month-old daughter.  The side seams are badly twisted on both sides, and the sack does not lie flat.  It looks like it was either sewn improperly or washed or dried at high heat.  Other reviewers stated that items made in certain countries were of lesser quality.  The one I received was made in India.  These have gotten such great reviews, and I really don't want to use a polyester sleep sack, so I plan to order another one in the hopes that the next one won't be defective.

UPDATE: I just received a second sleep sack after sending the first one back.  I now know that the first sack sent to me had been washed and/or dried improperly and then returned.  It was about half the size of the new replacement, and came with no instructions or tags.  The wool on the inside was also pilly.  The reason the seams were all twisted and wavy was because the garment had shrunk but the seams hadn't.  There was no way it would still fit my baby at two years old.

The new one was quite large and laid flat, and came with a pretty mesh bag as well as instructions for use.  Unfortunately my baby hasn't been weaned from swaddling so I haven't gotten any use out of it yet.  I tried putting her down for her nap in the sleep sack and she wouldn't stop crying.  I gave up and swaddled her instead.  I will update this once we're able to wean her from swaddling (hopefully soon).  I will also be very careful about how I wash and dry it, since I know what will happen if I do it wrong.