Overlapping regions and change of font size when a document is opened on computers with different DPI settings.
Everyone could change their dpi settings to the standard ones as explained for example here :
http://www.lambers.com/support/dpihelp.html
However, depending on your monitor this type of changes may kill your eyes, not always standard dpi is the best configuration for your screen.
For now, you may be forced to change/guess the proper dpi every time a person shares a worksheet with you.
SMath should track the original dpi with which the original worksheet was created and deal automatically with dpi settings for you.