I was once in a meeting on site. The task was to get some data from an AS/400 to a Linux server and display it on a website.

Text files would have been enough to cache the data. But you never know how a project can change over time so I proposed PostgreSQL or IBM DB/2. The latter in case they absolutely wanted to pay for a database and they are already using an IBM AS/400. (We were using PostgreSQL and Informix.)

At the meeting they said they have already bought a license for an Oracle DB. Without consulting us. So we were forced to use it without prior experience.