It's the 2005 San Marino Grand Prix at Imola. Pre-race inspection by the FIA stewards showed that the BAR 007 weighed in at 605kg, the minimum, but legal. In the post-race check on Buttons car, however, the stewards discovered a little secondary fuel tank which, after draining, left the car six kilos below the minimum weight. Illegal. The FIA was infuriated and disqualified both BAR's from the classification and the following two races. What did this secondary fuel tank do, or meant to do?
BAR-Honda 007, Wikimedia Commons
Pretty simple, actually. With the extra six liters added, the car was at legal weight at the start of the race. During the race however, they would burn up the fuel from the secondary fuel tank, making the BAR run six kilos lighter than the competition. During the last stop (often deliberately scheduled late in the race) the team would top the secondary fuel tank off, bringing it back at legal weight. With this system, the BAR's were able to run lighter than the competitors for at least half of the race. BAR appealed to the FIA's verdict. They could have gotten disqualified for the entire season, but the FIA had no evidence showing any structural and deliberate cheating. BAR's explanation? They needed the extra six liters to get the Honda V-10 started. Smart.
The Honda R005E 3L. V10 engine, pushing out 900 bhp. Wikimedia Commons.