When he let Cell achieve his perfect form.
Cell, Dr. Gero's bio-engineered Android that is built specifically to destroy the Z-fighters (including Vegeta) and take over the planet, is on the ropes. Thanks to the Hyperbolic Dues Ex Machina Chamber, Vegeta has packed a year's worth of training into a day, has become super buff, and is squashing semi-perfect Cell like the bug he is. But, in a moment of sheer stupidity, despite the warnings from everyone around him, Vegeta, in chase of a "more thrilling battle," lets cell absorb Android 18, thus achieving his perfect form. Cell then proceeds to wipe the floor with him, and Goku has to step in to save his dumb ass.