As said in the post preceding yours: In this case it wasn't a problem inside the sketch, it was the sketch's attachment.
Concerning the general remark with errors inside sketches: If you look at old posts, many people used to center rectangles by adding two symmetry constarints to a vertical and a horizontal side. The sketches looked good, and could be padded. But all of a sudden two or three features later weird errors occured with invalid geometry. That's why I started to write some notes about sketching, which finally ended in a document of 80 pages.
The current sketcher handles such issues much better.