Revealing mistake: After the crazy man cuts his own hand, he laughs and waves the knife. Watch carefully and you can see the hose behind the blade the blood came through. (00:13:40)
Revealing mistake: When Leatherface is cutting up Kirk while Pam is on a meathook behind him, he cuts Kirk with the chainsaw twice, but when he lifts it up there is no blood on it. (00:38:20)
Revealing mistake: Toward the end of the movie, when Sally escapes from the Leatherface family in the dining room, she runs and jumps through a window. As she does, it is blatantly a stunt double; note the curly blonde wig. (01:15:20)
Revealing mistake: The position of the hitch-hiker, in relation to the lorry, as it hits him, changes between shots. Also, the hitch-hiker is an obvious dummy as he is hit; you can see the fake limb attachment on his arm. (01:16:25)
Answer: The yellow shirt guy keeps running off screen while the main character jumps into the back of the truck. Since Leatherface's attention was solely on her, it's assumed the guy survives seeing as no one left in the Family is in pursuit.