This happens near the end of the film. The terminator is in the truck and riding after Sarah. Reese blows the truck up before the terminator can catch her.
Watch quite carefully now. The terminator stumbles out of the right hand door of the truck (the driver's side), falls down and rolls over.
Sarah sees this, and walks past the terminator and around the truck. The whole time though, she has her eyes on the terminator, only looking away when she finds Reese.
She hugs Reese, and after a few seconds, the terminator rises up, covered by rubble. Remeber that they are on the side of the truck opposite to where the terminator stumbled out.
Firstly, how could rubble be on top of the terminator if it stumbled out of the truck and then even rolled over.
Secondly, when the terminator stands up, it is on the other side of the truck (ie opposite to the side it stumbled out of). This is impossible.
You might argue that the terminator crawled there, but Sarah was watching it for most of the time before she sees Reese. Also, even if it could crawl over to the other side of the truck so quickly, surely Sarah or Reese would have heard it. It was damaged and made a noise as it moved, not to mention the scraping along the ground.