A possible issue may occur via 2 elements a line and arc that meet. Both are the same weight. Create a pdf using the pdf.pltcfg plot driver. In the pdf the 2 elements have slightly different weights. Using a windows driver to print to paper works.
This appears to be an Adobe Reader or Acrobat issue: the "problem" does not appear if you turn off the "Enhance thin line" option in "Page Display" in the Preferences.