Do a search on this, there are at least a couple of threads. Even with a delay noted, this can change back to the original departure time, or in between the original departure time and the delayed time. It's happened to me before, and my GF nearly missed the last flight of the night because of an incident like this. UA will tell you to be there on time, even if the flight is listed as delayed.

As I'm sure I've posted before in the previous threads you an search for, there is a customer committment item you could probably use against UA if you are affected by this. It says something to the effect that if a delay has been announced, because passengers might choose to leave the gate area until they need to be there for the new departure, the flight won't leave until the new (delayed) departure time. UA doesn't really follow this, though, in my experience. Especially at the hubs, a late arriving aircraft, or one that goes MX, could be swapped to another aircraft ops find is available, causing an announced delay to be rescinded. This can also happen if a mechanical problem is fixed faster than normal. There are probably other things that can do this too. Another possible situation: a delay due to predicted weather, but this can change frequently, and the window for departure can be fairly short if it is a break in weather.

I'm not saying I necessarily agree that delays should just be able to be rescinded, but, am saying that it does happen.