Very often, even not only beginners make the same mistake when using the standard Mirror tool.
If you use the standard Mirror tool from the Main Toolbar, when mirroring objects, all polygons of the new object become "Flipped ".
This cannot be seen in the Viewport until you do Reset XForm and enable Backface Cull in the object properties and enable the Clay display mode (it all depends on the Viewport settings, but it is better to use the Clay mode).
Try creating a Teapot object in a blank scene, change the Z-axis Scale value from 100.0 to -100.0. A scale value with a minus value mirrors the object along a specific axis. It seems like everything is ingeniously simple, but after such actions your polygons will also be flipped out.
Using the Symmetry modifier.
Great option, especially if objects need one Mesh. If the objects need to be different Mesh, then you will have to Detach them.
Using the Mirror modifier.
The advantage is that the settings are as similar as possible to the regular Mirror tool. But the objects also remain one Mesh.
Use the same standard Mirror from the Main Toolbar, but then apply Reset XForm to the new object, and then manually flip all polygons.
And for third-party models (for example, from stocks), I always do Reset XForm, my script always helps me with this Stock Model Fixer .
It can also help fix broken geometry with Snapshot, Detach Trick and Box Trick tools, and reveal problems with the model using the tools in the Check tab.
What methods do you use? Do you pay attention to Flipped polygons in your work?
Write in the comments, it will be interesting to know!