well, this time I report a strange behaviour occured under Windows XP release.
I was trying to check some corners that were not "joined": I mean that the two lines behaved like separate lines.
In fact zoomming in with mouse wheel I saw the truth: there was a very little gap between the 2 lines.
Just to better see what I was doing I continued to zoom the part but... suddenly one line became invisible!
I zoommed out and... voilà! There was again.
I zoomed in again, but it disappeared again.
Well, I had to got pleased with the latest visible "frame"...
However if I did left click around (when the line was invisible) I got it: I could see in the property editor panel that I caught it.
This is not the first time I meet this problem and always under Windows (but another XP release-now SP2"- and another hardware configuration).
The same drawing didn't cause the same problem under Mac OSX (Snow Leopard).