A couple of years ago the PM for the project I was working on committed to a 3 week deadline on a HUGE ASS project. The client was a massive telecommunications company and the project consisted on the websites for over 30 countries. When I confronted him, he told me he needed PROOF that the project wasn't going to be done in 3 weeks (WTF!?)...
He made the design, front end and back end teams start working in parallel (WTF!?)... Needless to say, the project wasn't done in 3 weeks, or in 10, or in 50... this was in 2014... the project hasn't been finished yet. Thankfully, I managed to get off that ship after 2 hellish months.