Government IT - enough said
Biggest IT SNAFU I've been involved with? Put it this way - I used to work for the Office of the e-Envoy, responsible for UKOnline and the Government Gateway.
Microsoft involvement meant that the portal required IE, for 'security' purposes. After a couple of months I think we managed to get it to work, mostly, in other browsers, but initial signup still had to be in IE on Windows.
Later we discovered that the terms of the contract meant that MS got the rights to take away all the things it had learned from working with us to set up the government gateway, and implement the same thing elsewhere, including stuff we'd developed in house.
That whole wretched episode isn't entirely deleted off my CV, but its appearance is brief...