BRU is obnoxious about this. You also need to go to the sticker people if you are connecting Schengen to US, and I once forgot and almost didn't get to board because they weren't reminding people until they were trying to board. I was one of many. But also, the last time I was there last year (and this was BA or SN, not UA, I think), something did not check out and they made me go through the sticker line at the gate twice. They finally figured out that the check-in people had entered something wrong when I had checked in. After they figured it out and fixed it, I did go to the front of the (by then very loose) boarding line. Someone did say something and I replied that this was my second time in line and I was way ahead of them. They let it go.