To reproduce -
- Make a rectangle sketch and Pad.
- Select a face and create new sketch.
- Attempt to pick and edge or vertex as external geometry.
It seems that the eternal geometry is being added, but is not being shown, (no edge or vertex shown to be selectable). Close the Sketch and reopen it, the external geometry is shown and works as expected. Something funky going on...

Mark
OS: Kubuntu 14.04.2 LTS
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.16.5111 (Git)
Build type: Release
Branch: master
Hash: 238becaebb616db39e8836763fdbfc468a8a7fc5
Python version: 2.7.6
Qt version: 4.8.6
Coin version: 4.0.0a
OCC version: 6.8.0.oce-0.17