A really easy example is the "Two generals problem". We know (mathematical proof, and frankly you can work it out with some thought and no training) that you can't achieve the reliable agreement required in this problem. But it appears that many every day things require such agreement. You can't create a TCP connection without it, for example, or buy something with a credit card.
In reality what happens is that we put up with a negligible chance of failure. The failure of this sort are vastly outweighed by more pragmatic problems (imagine if the generals managed their agreement, and then one finds his men have developed dysentery and cannot fight) so we don't worry about it very much.