In preparation of the total solar eclipse that just happened in August, I bought a EOS 500D from eBay. Everything was in excellent conditions except that I always saw a defocused view from the viewfinder by eyes, although the images were at focus. What is more, the boxes and circles around the AF points are slightly off from where they are supposed to be. The image below shows what I saw through the viewfinder. The centers of all the boxes are slightly above the corresponding AF points. A horizontal line can be also seen at the bottom of the view.
After some exploration, I found that this issue is because the dummy glass plate that contains these boxes are displaced. Please see the below image. A very thin metal frame holds this dummy plate above the mirror. This metal frame is locked by a small metal tooth (green circles). In my case, the engagement had fallen off. I used a tweezer to firstly pick up the tooth and shake the camera gently. This moves the dummy plate to the right position. Then, place the metal frame back and engage the metal tooth using the tweezer. Now when I look through the viewfinder, everything is correct!