I've come across this scenario: when offsetting a line (while in a rotated UCS) the distance between the "parallel" endpoints differ by fractions of a unit (I'm working in millimeters so don't draw in fractions).
This seems to happen throughout all the drawings, only when operating in WCS does this not occur. Usually it's not a problem, as the variation is very slight (something like 0.01%) - but the error gets compounded over large buildings where you've offset many times. For example setting out a grid at 6000 intervals sometimes gives an overall dimension (over 18 lines) of 108003. Then I have to adjust each line slightly so their dims don't show something other than 6000, while the overall should show 108000. I know that you can't find a contractor to build at that accuracy, but I'd at least want my drawings to appear as if I've had a calculator at hand.
Is there some error in AutoCAD? I know that you get inaccuracies when using the Copy-n-Paste functions, as well as using the Express Tool's Flatten command, but these commands were not used in the problem drawings - exactly for the inaccuracy it creates.
The only thing I can think of is that there's a slight loss in accuracy when converting between WCS & UCS. Possibly because of a different floating point variable type used - something like single precision instead of double (or extended).
Has anyone come across this before, if so have you found a solution?