Acrobat bug - 10 inches
Usually, when I scan a document, Acrobat retains the settings from the previous scan.
Except when the measurement (either length or width, doesn't matter) is exactly 10 inches. In that case, when I go back the measurement will reset to 1 inch.
This is easy to work around by using measurements like 9.99" or 10.01", but I recently changed computers and operating systems and the problem persisted, so I figured it was definitely a bug on the Adobe end, instead of on my end.
