"No plan survives contact with the enemy." - Helmuth von Moltke the Elder

Some clarification for readers: this generally doesn't mean that a plan is scrapped entirely; it means that changes are normal and anticipated (i.e. NOT an automatic sign of failure) that as new information comes to light and circumstances shift, those executing a plan should be adaptable enough to make adjustments where prudent. In fact, decentralized command was developed in order to allow highly-functioning teams to actively exploit unforeseen advantages which develop as a battle/campaign unfolds.

Although these concepts were forged on the battlefield, they are 100% relevant to civilian 'battles' as well - in this case, time management.