I was keeping an eye on a friend's flight on PlaneFinder. It was meant to be flying past my office, so I walked to the window to see (the airport is about a minute away when they pass, so it's pretty low). However it was about two minutes later that the plane went past.
What gives? I can't see flight sites showing it in advance, but on the map it was showing it low over Richmond, Vancouver and inline with my building - it would be incredibly hard for me to mis-read the map, but I can't think of any other option.