Cracked and dented forcing cones
Out of time
Side plates that were so poorly machined that there were significant gaps between the plates and the frame
Barrel installed out of time so the front sight was significantly leaning towards the 1 o'clock position (This was an "L" frame that was returned to S&W. It was returned untouched with a note stating that it was within "acceptable" specification)
Significant gaps between the frame and cylinder crane
Pinged/deformed hammer noses
Cylinder would not turn reliably (required replacement of sear and sear spring)
Cylinder would not reliably lock into place (required replacement of extractor star)
Variable gap between face of cylinder and forcing cone - differences of up to .005" observed, and cylinder would hang up at "high" spot
That's what I recall. I'm certain I missed a few things. These issues occurred to guns over a period spanning from 1973 to 2009
In addition to the aforementioned "L" frame issue, I returned two separate "N" frame revolvers to S&W for different issues. Neither issue was resolved
I took two different troublesome "N" frame revolvers to Frank Glenn, an outstanding gunsmith who knows a thing or two about S&W revolvers. After examination, he refused to work on either revolver and suggested I return them to S&W
In all cases the revolvers mentioned were either new or were nearly new when the issues were experienced
YMMV