"VVIP" parties are only too commonly the cause of overbooking issues, when the elite commercial team/Chairman's office staff responsible for liaising with them just trump the normal overbooking calculations that load management have devised, and pass any issues down the line to get sorted out. And yes, it is also common for such groups to chop and change their arrangements almost hourly - or even just turn up at the airport in parallel with the station manager receiving a message from HQ. Some of you will have experience of this.
Overbooking as such is a straightforward event, and load management generally know what is appropriate - even to the extent of beng 50 over a few days beforehand on a 747 from Heathrow (to India, in particular), and ending up going out with no empty seats and nobody turned away
. Any airline with a hub operation will know, for example, that they get many more no shows on the outbound legs (due to misconnects) than on the inbound; there are a wide range of other parameters. For example, if you sell single tickets more expensively than round trips, what else do you expect but that people going one way will buy a return and throw the second leg away. Those no shows are entirely due to your fares policy, just like the misconnects can be due to quoting legal but unrealistic and not always achieved minimum connecting times at the hub to get people onto your flights rather than the competition.
At least we are not in the USA, where excessive overbooking is a daily event. Not that this shows in any dneied boarding/downgrading figures, for the carriers just buy this out from "volunteers" at the gate. Not commonly done like this in Europe/Asia though.