Biggest scaling challenge I've faced?

Around 2006~2007 the business was in double-digit growth thanks to the eCommerce boom and we were struggling to keep up with the demand.

Upper IT management being more hardware focused and always threw more hardware at the problem. At its worst, we had over 25 web servers (back then, those physical tall-rectangle boxes..no rack system yet) and corresponding SQL server for each (replicated from our main sql server)

Then business boomed again and projected the need for 40 servers (20 web servers, 20 sql servers) over the next 5 years. Hardware+software costs (they were going to have to tear down a wall in order to expand the server room) were going to be in the $$ millions.

Even though we were making money, the folks spending it didn't seem to care, but I knew this trajectory was not sustainable, so I started utilizing (this was 2007) WCF services and Microsoft's caching framework Velocity. Started out small, product lookup data (description, price, the simple stuff) and within a month, I was able to demonstrate the web site could scale with less than half of our current hardware infrastructure.

After many political battles (I've ranted about a few of those), the $$ won and even with the current load, we were able to scale back to 5 web servers and 2 sql servers. When the business increased in the double-digits again, and again...we were still the same hardware for almost 5 years. We only had to add another service server when the international side of the business started taking off.

Challenge wasn't the scaling issue, the challenge was dealing with individuals who resisted change.