If you're receiving strange DFM error results (which you believe are incorrect), please Extract a Netlist (from the imported gerbers) and then run a DFM Check.
1. Go to menu: Tools > Nets > Netlist Extraction...
2. Review options and press button Next to start extracting a new netlist. (typically 15-60 seconds)
Now that you have a fresh netlist generated from the Gerbers and Drill, go ahead and run a DFM Check; and those strange DFM errors are resolved.
Why/When should I Perform Netlist Extraction:
In many situations the PCB CAD tools do not output accurate/optimized CAM files -- which is why it's imperative to perform DFM on those CAM files before manufacturing - since these CAM filesare used to manufacture the Boards.
Screen capture of random DFM error:
Same data viewed with Fill Off (in FAB V9):
Note: You'll notice a large polygon (without a net) that randomly overlaps all the geometries. When filled everything looks perfect and unified - however when the fill is off - you'll see exactly what objects and resources the DFM Engine is processing. Performing a Netlist Extraction on the CAM files will ensure the correct nets are generated - resulting in much better DFM Results.
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A majority of PCB verification and manufacturing issues can be attributed to the quality of the CAM files being used. The gerber quality output from many PCB Design tools (commercial and especially open source) have been notoriously incomplete with many formatting and attribute errors, overlapping objects, etc. Here's more information on that topic, https://support.numericalinnovations.com/support/solutions/articles/14000168091-why-should-i-perform-a-dfm-check-
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