At Brussels airport they have electronical gates that will only let you through if you scan a valid boarding pass. After the gates, you go immediately to stand in line for security control. In that line, you have to show your boarding pass to a person who scans it with a handheld device.
I have not experienced this in any other airport, yet is has happened every single time at BRU, for the three years I've been coming there. Why do they have a person check exactly what the machine did, directly before? It seems like a very unnecessary waste of work.