One of the biggest and most expensive was using Cassandra to store membership details. Something like 4 years of work, by a team of 40, wasted by stupid decisions.
They included: o Using Cassandra to store 6 million rows of highly structured, mostly readonly data
o hosting it on real tin, expensive tin, in multiple continents (looking at >million quid in costs)
o writing the stack in java, getting bored, re-writing it as a micro service, before actually finishing the original system
o getting bored of writing micro services in java, switching to scala, which only 2/15 devs knew.
o writing mission critical services in elixir, of which only 1 dev knew.
o refusing to use other teams tools
o refusing to use the company wiki, opting for thier own confluence instance, which barred access to anyone else, including the teams they supported