Network Designers: Have you ever (or witnessed someone who has) bought the wrong gear for a BIG project? What were the results?

I'm talking about overlooking some critical gotcha, like a required feature not being supported, lack of compatibility with existing stack, or vastly under-spec'ed gear for the load needed, etc. Things along those lines. Things that would make you wonder "what were they thinking?" Things you'd just say "that isn't going to work!"

Share your story, what you/they did wrong, and what the end results were?