In Jurassic Park (1993), the T-Rex animatronic would sometimes randomly “wake up” on set because rainwater messed with its hydraulics, startling the crew between takes.

In Jurassic Park (1993), the T-Rex animatronic would sometimes suddenly “come alive” on set.
During rainy scenes it absorbed so much water that the hydraulics glitched, causing the giant dinosaur to move unexpectedly and freak out the crew between takes.