Signature images are written to disk every 30 minutes. So it's possible that the wrong units were used to write a particular image to disk and they stayed that way for 30 minutes while you were testing things out. Then by the time I brought up the image everything was ok.
There definitely could be a bug lurking in there. I don't see anything wrong with a quick glance, but let me know if you run into it again.
DTMAce 
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