Hive scene in Aliens, when the Warriors descended from the walls. Watch their tails move. Instead of curling in/out, it just moves like a stiff board. In the shaft when they're chasing after Vasquez and Gorman, they just kind of pop out of the walls like whack-a-moles as opposed to the Aliens we see in AvP which are crawling in and out of crevices.
In Alien, the scene where Lambert is supposedly raped. When it turns around to nail Parker, it's arms remain in the same "hugging" stance and it looks like it's on a swivel when it turns. And the last scene right before Ripley turns it to ashes, it just kind of floats there. No movement, nothing.