Weird indeed but moving the prism up 1mm solves the problem.

Dan - the bug came immediately after:

if e.is_a? Sketchup::Edge

... it was very strange, and as I said, happened once in a great while - I couldn't figure out how to make it happen on command.

Dear Tim,
Im an old fart just looking for old school equations for lamella roof construction, ie formulae for timber member outer radius , length ,end bevel angles etc given a roof width and height, cant find anything under Zollinger or Junkers.
Regards,
Fred

Thx ..my carelessness in thinking a vertex was a drawing element! .../T