I don't agree with 100-1/2's interpretation, and neither does our POI. There are no duty time requirements once you are done flying; the only requirement is that you get 10 hours of rest (or more, if you went over your flight time) before you fly again. Duty time applies to flight operations only.
POIs recognize that there is a little bit of duty after completing a flight, and would suspect a situation where the manifests show departure exactly 10 hours after your last arrival, but I have never heard of any hard-and-fast 30 minute rule. I have heard that some POIs consider any time at the airport "duty", but I would defy them to consider an early-morning joyride in my Taylorcraft disqualifying for a trip 9 hours later.
We send people on multi-leg trips with stopovers that our software predicts as 13+55. Sometimes the 14 hour limit is exceeded, but I would be comfortable looking my POI in the eye, handing him the dispatch paperwork, and saying "but it was planned for less than 14", and I fully expect that he would say "OK." I would be a lot less comfortable without the dispatch paperwork, however. When limits are inadvertantly exceeded, we file a report with the duty time logs.
Typical examples for us involve unforecast weather: depart with a forecast of visibility 4, arrive with visibility 1/4, divert, wait for truck with the freight to drive (slowly) through the fog, and continue. It's helpful if your preflight paperwork includes a printout of the relevant TAFs.
The three most important things (from the FAA's point of view) in commercial flying are paperwork, paperwork, and paperwork. When in doubt, confuse them with the truth.
Duty time regulations are hard to understand and hard to interpret. We spend a lot of time with out new hires on this, with the charter coordinator and more senior pilots as very needed backup, and we still see a few mistakes. These pilots get additional training and perhaps a friendly line check from me (I am a check airman).