I mean how ? Why ?

1) You develop a feature, covers every case where it can go wrong. Dry run 2-3 times. Then comes the day of UAT meeting. You are displaying feature in front of all stake holders and the feature breaks. 2) You are trying to solve a bug. You tried every way there is which you can think off. You are scratching your head, figuring out why ? Now you finally decide you need big guns, ask colleagues/seniors for help and you explain them the issue and you also explain them the solution which you were trying. Now magically issue is solved or you atleast understand what was wrong and why it was not working. What phenomena is this ? Does this only exist to make you look stupid ? How ? Why ? There got to be some explanation for this.