What were your biggest purchase mistakes?

[Lance]

I got a good price on a new Aqualight Pro 3x 150W HQI, 4x 96W CF and 4x LED moonlights. I had no issues with the fixture, the problem was me. The tank was going to be a FOWLR and the light was plenty good enough. I soon changed my mind and turned the tank into a reef, making the light underpowered for corals. So I had to buy another light.