Some tasks may simply not be possible or not in the time available. Agile just sort of tells you that - you can see how things are going and if they're going right or not.

Then you have a choice - find something to cut out or accept a later date. This is a mode of thinking that I find non developers have difficulty accepting. They want it all and they want it now and their modus operandi is to keep pretending that it's possible and suggesting that if they shout and stamp a bit that it will somehow rescue the situation.