After Heddy receives the phone call from her father, she hangs up and takes the phone off the hook. As she walks away, a busy signal is heard. Heddy yanks the phone out of the wall, yet a busy signal can still be heard.
The goof items below may give away important plot points.
When Allison is strangled in the elevator, there is a small hole in her shirt where she was shot a couple minutes earlier, but no bloodstain. However a few minutes later a bloodstain appears and her arm is fully functional to fight Hedy.
After Hedy tied up Allie and Hedy leaves for the bank, Allie turns up her TV to alert the neighbors as a call for help. One neighbor opens his door and gets the landlord, all while Hedy is out. The neighbor and the landlord are at Allie's door, the landlord fumbling through the keys, yet Hedy comes all the way from the bank, walks back to the apartment building, waits for the elevator and takes the elevator up 12 floors, and the landlord is still trying to find the right key to open Allie's door.
Apparently, the huge NY apartment complex where Allie lives has only four tenants: Allie, Graham, the landlord and the neighbor who brought the landlord down to open Allie's door when she turned the TV up too loud as a call for help after Hedy had tied her up. Gunshots, broken items thrown about, violent struggles - all were not loud enough for the one neighbor to complain, but they do complain about a rock music video. No other tenants are seen in this highly desirable section of the city in which to live, and the building looks all but abandoned. Additionally, the ventilation system that Graham (Allie's upstairs neighbor) pointed out carried sound so well did nothing to alert other neighbors that perhaps somebody should call the police.