@tig said:
Simply, something is wrong with the SKP's database.
Usually it's Lines with zero length, or Faces with zero area, but it can also be incorrectly 'named' objects like Layers etc...
In the case you show there was a [useless] tiny face with a 'zero' area, which it erased.
SUp has 'tolerances' so that two vertices within a range are deemed to be coincident, otherwise you'd never get anything to form! Thus very very short edges can be thought of as zero length, which then produces a zero area face... Sometimes things fall through the net...
In this case if you were making a Followme extrusion of a profile that had some tiny edges and it went around a path with some tiny segments, then quite often it just fails to make those 'too tiny' faces (<~0.01sq") and leaves a hole in the mesh; in your case perhaps it actually made a tiny face that was effectively zero area, but after auditing the database and realizing it, SUp erased it - as it's effectively of no use to you.
holy cow!!! seems like you've extracted all the coding of sketchup!!! you really know all the nuts and bolt assembling of sketchup working!