Several aliens are seen being killed numerous times. The two aliens playing patty-cake are seen dying at least four times each (both of them during two same scenes). The first alien shot at the beginning of the gun fight also falls from the tree.
After Derek jumps on top of the lead Alien with a chainsaw, the chainsaw falls to the ground between the Alien's legs. The camera pans back to the Alien who does not move, however in the next shot the chainsaw has moved a few feet away.
When Derek closes the flap of his skull the second time just before he ties his belt around his head, his hands are upside-down. If he were reaching around to the back of his own head his thumbs would be on the bottom. They are on the top.
When the boys are getting kitted up in the forest to rescue Giles, even though there's supposed to be nobody for miles, through the trees in the background you can clearly see cars driving along a busy road.
Obvious mask for the aliens at the end when Derek goes through their head. The 'alien' heads have no skull, when the head goes in, the skin gives-in and reveals it's just a mask. Also at the very end when Derek uses the alien head as a 'mask', it's obviously a regular mask with made of plastic.