As a junior in a print communication agency, my boss wanted me to make their portfolio.
Their requirements were: a full animated flash website (in 2010...). Understand, they had been bought the Adobe license...
After several months of works and ton of alerts about flash death, the website has been deployed.
My boss did not understand why he could not visit the website with its iPhone...
The website had lived 2 months and will was replaced by a static "wix" alternative... So much work for nothing because the boss did not trust a junior dev.

Biggest lesson: Always begin with fast proof of concept to validate your hypotheses for you and for your boss ;)