On Friday evening I was traveling back home. The last leg from FRA to FDH is with a regional jet which always is parked in an outside position and requiring a bus transfer from the terminal. I was happy because the bus was leaving the terminal 20 minutes before the departure time, usually a sign of on-time arrival (that's what counts). However, it turned out differently...
Once we arrived at the airplane, all the passengers boarded and took their seats. We then waited for the usual "boarding complete" and welcome message which didn't come. Instead, the flight attendants counted the passengers, re-counted, compared their passenger list with the seated crowd, and then got busy. At first, they asked the passengers in business class to show their boarding passes. After some more discussions, they went around in economy class and asking for specific passengers. Suddenly, at least for us boarded passengers, two more passengers were brought to the plane and also another car from operations arrived.
With some more discussions between crew, operations, and some passengers, the puzzle eventually got solved: It seemed that the crew was handed an outdated list (which they discovered) as there were some last-minute changes. Some of the new passengers arrived on time for boarding while some others on the first list arrived late. Hence we had a mix-up in the passenger count and with some names. All this caused a delayed departure of 30 minutes (and 20 minutes for arrival).