Haven't seen a similar issue in the bug tracker, so I'm posting here.
Steps to reproduce:
1. Open the attached file:
2. Change a dimension in Sketch008 placed inside Pad002 (eg 139 to 140)
3. Now Sketch_LCD (inside LCD pocket) is invalid (red exclamation mark)
4. Remove external edges from Sketch_LCD, now "Sketch contains conflicting constraints" message is visible.
5. Close the sketch.
6. Try reopen the sketch. Now "Do you want to open the validation tool? The sketch is invalid and cannot be edited"
7. Open the validation tool.
8. Find and fix 4 missing coincidences.
9. The sketch is still invalid, but now tooltip in the tree view says "No planar face in the AttachEngine3D::calculateAttachedPlacement()!" instead of "Error"
OS: Ubuntu 17.10
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.17.13050 (Git)
Build type: None
Python version: 2.7.14
Qt version: 4.8.7
Coin version: 4.0.0a
OCC version: 7.1.0
Locale: Polish/Poland (pl_PL)
I made this submission to help an user in the Polish subforum https://forum.freecadweb.org/viewtopic. ... 68#p210168