Why didn't the pregnant woman untie the others?
Ok this really bugs me. Martin never bothered to duct tape the pregnant woman's hands. Sure, she's far along in pregnancy and wounded so she's pretty weak but she was left alone with the other victims for at least a day or so. Surely she could have mustered up the strength to remove the tape from at least one of the victim's hands. For that matter, none of them were taped very tight, you'd think they'd be able to wriggle free given the amount of time the movie implies they were left alone (anywhere from several hours to a few days)
Also, no one noticed all the blood in the parking lot? That sort of thing tends to attract attention and Martin did all of his kidnappings on camera.
Honestly, this movie reminded me of an old Mystery Science Theater 3000 episode where they gave out the "They Just Didn't Care" awards.