Had two day long interview processes. Was interviewed by over 10 people across 5 interview panels. Offer was extended on day 1 . Post 15 days of making the offer, the HR asked to come and have a "discussion" with one of the directors, which turned out to be another round of 3 panel interviews. While the director in question was positive and looked forward to working together, the HR informed that I would not be suitable for the role. Thoroughly unprofessional of an organisation of this scale to extend an offer and then withdraw without any reasonable explanation