What is the most cunning cheat in F1 history?
What is the most cunning technical cheat in rules ever seen in Formula 1?
This is a matter of opinion, but there have been quite a few tries at bending the rules. In the 1980's there were turbo engines and regularly aspirated ones and it was allowed to fill the water tanks of the cars after the race. At this time many cars featured huge water tanks that were spilled during warmup lap and then filled again before final weighing.
Another one was a pneumatic system for altering the ride height of the car. The car was lowered for the race, but after the race it slowly rose up to the level required by regulations.