The vanishing cylinder bug trick.

 From:  DannyT (DANTAS)
Hi Michael,
This one almost had me baffled, but I managed to get it to reproduce.
The file attached is at step 4. before the fillet, when you fillet this edge the cylinder vanishes.
The procedure was:
1. Offset surface 3mm

2. Extrude cylinder from circle.

3. boolean diff surface from cylinder

4.Fillet this edge 1mm

5. Hey presto! vanishing cylinder