I searched the forums for this issue, and it's identical to this thread: viewtopic.php?f=7&t=2486 I just started a new thread because I didn't want to necropost...
So obviously, either the glass bed is warped upwards or the x-axis is bowed downwards or both. I figured the glass was just warped, so I flipped it upside down and clipped it, then checked the gap. The left and right sides were still lower than the center. I checked the flatness of the glass with a stiff metal straightedge and (carefully) laid it edge-wise across the bed at multiple locations and angles. I put a light behind it to more easily spot any gap between the glass and straightedge. I found that if anything, the center of the glass was actually bowing down, not up.
Next, I placed the straightedge across the printer's x axis rail. Sure enough, there was contact at either end, but there was about a .1 mm or so gap in the middle. If I knew nothing about the precision necessary in this hobby, I'd have thought it was nothing to worry about...
I've thought about putting shims underneath the x-axis rail and/or loosening the x-axis belt tension (which I haven't touched since I got it - the x and y belts have roughly the same tension).
Also, I've noticed that the aluminum heating plate underneath the glass bed is very noticeably bowed downwards by 1 to 1.5 mm. The axis of bowing is perpendicular to the x-axis bowing - that is to say, it's as if the y-axis of the aluminum plate itself was bowing downwards. However, the y-axis of the printer is perfectly fine. Anyway, the glass only fully contacts the heating plate at the front and back edges. I can physically press the aluminum and glass plates together back and forth along the center where the M2 logo is. With the bed clipped at the front and back edges, I can slip 3 index cards between the glass and aluminum at the left and right edges
![Surprised :o](./images/smilies/icon_e_surprised.gif)