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

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.
 
Flying back to Dublin from Portugal last month on a late flight this happened with Aer Lingus - not overbooked but one of their cabin crew was unwell and they have to have a passenger/crew ratio - needed 30 people to wait until the next day - they offered hotel & €400pp who would stay. They managed to get the 30 people after a lot of hassle. In fairness to the crew at the gate some of the passengers were very abusive and others were videoing the whole thing on their phones.... I expected airport police to come but it eventually got sorted out.
 
Coming back from London Gatwick last weekend from the rugby and several people weren't able to be allocated a seat when checking in online. They had to wait to he allocated at the gate. I think 5 or 6 didn't get on. A mate of mine did. The plane was over booked. Ryanair. At the Gate they were waiting for no shows/late shows and then re allocating their seats.
 
Happened once in Aer Lingus flight coming back from Zurich. We (two of us) were not allocated seats and were told to wait for no shows. We requested to ask for volunteers instead. Luckily a couple came forward to stay another night in Zurich paid by Aer Lingus plus vouchers.