As reader AlexPGilbert pointed out in the comments for the Isometric Picking Tutorial, the tutorial code contains a bug that shows up when the camera is offset vertically by an odd amount. I have updated the tutorial with extensive changes to the Camera class (That I had planned to include in Part 8 but actually fix the problem with the misplaced hilight). The download Zip file has also been updated.
There may still be one issue with the code, but I'm working that out as I finish up on Part 8. It is fixed in the version that is online now.