It was early 2013 (or end of 2012). C model machines, only. Have only found that a couple of machines had this issue.
Z calibration needed to be reset via a file specifically created for that machine serial number. It was made obvious when carving centerline, but happened EVERY time, not intermittently.
Carve a 1" x 1" square carve region set at .25" then measure the depth of the carve.
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