I came across this topic through Google. I bought Yashica Mat EM on eBay just two weeks ago, and today I developed the test roll. It turned out that all frames are completely out of focus although I carefully focused each shot by using the magnifying glass. Camera was steadily on tripod.
When shooting the test roll I noticed that I wasn't able to get a subject that was approx. 1 meter (3.3 feet) away in focus -- although 1m is the closest focusing distance of Yashica Mat EM. I had to take the camera approx. 2 meters (6.6 feet) away from the subject to get it in focus. The focusing knob's scale was at 1m at that time. Also the frames that were focused farther away were out of focus.
I'm assuming it has to do something with the ground glass because the camera is in top shape. The seller says he has been using the camera without problems. The seller says that just before the camera was listed on eBay, the camera was checked by a camera store. The camera store may have broken the focusing while cleaning, right? The screws that keep the hood+glass attached to the camera body are scratched, looks like someone (the repair man) has unscrewed them recently.
The camera arrived from the UK and I live in Finland, so complaining at the camera store is not possible. I just need to get the camera checked and fixed in Finland. It's a shame, the camera is really nice. Like new.
// edit: I just popped up the hood again and put my finger on the focusing screen (is the top glass the ground glass?). I am able to move the top glass both vertically and horizontally. I'm pretty sure this shouldn't be possible. Missing spacers/shims?