I worked at a place in love with their ERP system. Some there had been using it 30+ years, since it ran in DOS.

My Excel skills completely blow, and I hate Microsoft with a passion, but I created a shared spreadsheet one long Saturday afternoon that had more functionality than our $80K annual ERP system. Showed it to a few more open-minded employees, then moved it to my server, never to be shown again. Just wanted to prove when I said the ERP system was pointless, that I was right.