There was an instance in rallying on the Tour de Corse, when both the works Triumph TR8s were tampered with in parc ferme before the start. IIRC someone had taken out the sump drain plugs. I can't remember if they were able to start (presumably there would have been oil everywhere). It was never expalined who might have done it.
Slightly OT, ther ewas the case on the Paris-Dakar when soemone stole the leading car, the Peugeot 405T16 of Ari Vatanen, during the overnight halt in Timbuktu or Ougadougou. They were caught driving it out of the city. How anyone could have thought that they could slip away in such a visible car is not recorded...
Triumph's case was the 1979 Tour de Corse; and it involved a discussion between Jean-Luc Thérier and Jean-Marie Balestre after Thérier won the 1000 Pistes.... as he was disappointed with Balestre's quotes on French drivers, he went to Corsica with a British license, and he was always convinced it was someone with FISA links that went into the parc fermé and sabotaged the gearbox of both TR7's