I live in the UK, and am watching a wedding show right now (Don't Tell the Bride, for any other UK knotties), and they had a tiered reception. Immediately following the ceremony, they did a sit-down dinner with the 50 or so people invited to the ceremony, and a dance/after-party with another 50 or so of their friends. From what I've heard from friends and seen on wedding shows, this is apparently quite common in the UK.
So, this couple's wedding day was running so late, that after-party guests (NOT invited to the dinner) were showing up at the reception location before the dinner even started, at the same time as some of the dinenr guests. The USHERS had to stand in the hall and tell people, "Wait a minute while we try to figure out who is supposed to be here...."
Talk about TACKY.