Hopefully somebody from the Upverter team will see this…I could not find another way to file a bug.
This is a bug that I found in the Gerber export of the bottom silkscreen layer. It appears that rectangles on the board bottom are being rotated in the opposite direction of the part, by the same amount as the part rotation. This is only apparent when the rotation is not increments of 90 degrees (since a rectangle rotated 90+90 degrees around its center appears exactly the same).
Here is a simple project demonstrating the issue: https://upverter.com/xpctr8/5bff150c1585d4df/Silkscreen-Rotation-Bug-Example/
The board linked above is flipped symmetrically so that the front and back should look exactly the same, except for the reference designations, and everything appears correctly in Upverter. However, when you export to Gerber and then view (I’ve tried Oshpark and http://www.gerber-viewer.com/), the back side rectangles are rotated in reverse. Notice that lines, text, and circles are rotated/positioned correctly in the export, so this only (apparently) affects rectangles.
Here is a screenshot from Oshpark demonstrating the issue: