PCB123 Technical Support recently worked an interesting customer issue we thought we'd share with PCB123 users.
One of the display speed options for PCB123 is the ability to turn off Rat-line optimization. By default, PCB123 does some compute-intensive work to display an otpimal (minimally confusing) rat-line. But doing so can sometimes slow down the screen rendering. So there's a user option to turn the Rat-line optimization off.
For this particular customer's design, they had turned rat-line optimization off. And at some point mid-way in their design, they switched from six layers to four layers. Some of the nets in the design had connected to the fifth and sixth layers.
The issue arose with these two items together - two removed layers that once had held nets, and rat-line optimization turned off. Had Rat-line optimization been turned on (the default), the unconnected nets would have been identified by rat lines. Somehow, this combination allowed the open nets to not be flagged for the user.
This is a pretty uncommon circumstance, to be sure. The "bug" will be fixed in an upcoming release. In the meantime, if you're turning off rat-line optimization while you design, we recommend turning it back on while you do your final check of your design before ordering.