Everything takes longer than you think. Anything that can go wrong will go wrong. If there is a possibility of several things going wrong, the one that will cause the most damage will be the one to go wrong. Left to themselves, things tend to go from bad to worse. If everything seems to be going well, you have obviously overlooked something.
Mother nature is a bitch.
It is impossible to make anything foolproof because fools are so ingenious. Murphy's Corrollaries Left to themselves, things tend to go from bad to worse. Murphy's Military Laws Never share a foxhole with anyone braver than you are. No battle plan ever survives contact with the enemy. Friendly fire ain't.