Revealing mistake: Look closely at the footage when the vampires are entering the cave. Later when they are exiting the cave, the waves are flowing backwards. The same footage has been reversed. (01:17:05)
Revealing mistake: When Michael is trying to keep up with Kiefer Sutherland et al along the beach towards Hudson's Bluff, you can see the tyre tracks from the camera vehicle in the sand. (00:21:35)
Revealing mistake: When Alan Frog is reaching for a stake off of Edgar Frogs back the stake is a "trick" stake. He pulls it facing upward and the tip falls down into the stake. As he lowers it off camera he flips it downward and then shows it again and the tip is back out. (01:10:35)
Answer: In the film, being invited in isn't a requirement to enter. It's simply a way for the vampire to render you powerless (i.e. the tests the boys performed on Max).