Rick,

I have seen designers use too small of a clearing on the inner plane layer 
pass-throughs. I have also seen mechanical mounting holes that did not need to 
be plated, get plated and cause a short. Newer DRC programs have helped reduce 
such problems, but they somehow still crop up.

A good net list test, run from good net list data can also help preclude 
defects. Whenever the layer count for .062" thick boards gets up to eight or 
more layers, I like to have the board shop run High Pot testing on the boards.