Has anyone been refused boarding on a flight they had been checked in for due to overbooking?

[Marsupial]

The advice from the staff was to buy a seat and check in as early as you can.

A no-brainer really.

And, if you don't buy a seat at time of booking, then another thing worth doing is to have a quick look at the seat availability plan when checking in online, and if there aren't many empty seats left for sale, then it's odds-on that the flight is close to being full (or overbooked) - so buy a seat at that stage.