I found some old bugs related to this on the google project page but they were closed or left dormat. There is an issue with rendering some jpegs. it ends up making them gray, blown up, and interlaced.
I say some because if I run 'convert some.jpg some.png' and then reverse that process 'convert some.png some.jpg' the image renders properly.
I also opened an affected jpg into GIMP and resaved it but without any EXIF, Thumbnail, or XMP data, I've attached a screenshot of the settings I used.
I've also attached a screenshot of the pdf rendering a jpg that has this issue, a jpg that I saved with GIMP and a jpg I converted to png and back via command line on linux. Top, left, Right respectively.
For now I can easily just run the convert command on these images, but it does appear there is a bug somewhere.
I've also attached a jpg that has this issue. Hopefully the upload process does not strip something off the image that lets it work on your end.
EDIT: I'm using 0.12.0 on Ubuntu 12.04. let me know if you need any more info.
Something I noticed in GIMP, is that the checkbox is always on for XMP data when resaving a jpeg that does not render properly, but is disabled on jpegs that render properly.
To test that theory, I ONLY unchecked XMP data when resaving a troubled jpeg, and it now renders properly.