Why does it take so long to realize you forgot something?

[jost]

On 7/23/2016 at 12:22 AM, something said:

Ariane 501 ended up as a 200M Eur firework show as a piece of software from Ariane IV was reused. During ascend the computer decided to be 90deg off course corrected that error and by turning the rocket in flight reading some strangely high g-rates the self destruction was correctly triggered. ###BOOM###

The most Kerbal of failures at the esa...

 

https://www.youtube.com/embed/PK_yguLapgA?vnddnc=1&vndapp=youtube_mobile&vndclient=mv-google&vndel=watch

 

They even had a simulator but management decided that it wouldn't be needed ( same software what can possibly go wrong bla?). After the crash it was used and the explosion was exactly reproduced:

https://en.m.wikipedia.org/wiki/Cluster_(spacecraft)#Launch_failure