Visible crew/equipment: When Casey tells the killer "Dial somebody else, okay?", you can see the shadow of some equipment on the decorative pillar at the top right of the screen as the camera moves backward. (00:03:18)
Revealing mistake: When Sidney in Ghostface costume stabs Billy with the umbrella, you can see the squared shape of a protection pad under Skeet Ulrich's shirt (which she misses the second time around, famously prompting a genuine scream of pain from the actor). (01:39:55)
Visible crew/equipment: When Casey agrees to play the killer's game she crouches down next to the television and as the killer says "Here's how we play..." you can see a clear shadow of the head of the boom microphone cast on the white wooden railing at the top right of the screen. (00:06:25)
Revealing mistake: After Billy's stabbing, Stu charges Sidney projecting her through a doorway. In that shot you can easily see that Matthew Lillard is replaced by a stuntman. (01:40:15)
Other mistake: In the first scene with Randy telling Jamie Lee Curtis to turn around, the reflection in the TV never mirrors the actual position of Jamie Kennedy. (01:22:30)
Audio problem: Gale exits the TV truck telling her cameraman "I'll be right back" to go with the theme of the movie. However, the line is dubbed over and Courteney Cox says something shorter and different. (01:14:25)
Continuity mistake: When Dewey and Gale enter the house party Dewey jokingly takes a bottle of beer off a party goer then gives him it back. The bottle is back in Dewey's hand again when he speaks to Tatum. (01:03:40)
Revealing mistake: When Sidney is attacked by the killer for the first time (in her house), she is knocked to the floor and you can see she is played by a stunt double. (00:28:39)
Continuity mistake: After Casey has been stabbed in the chest she falls down and the killer gets on top of her, before he can stab her again she knocks the knife out of his hand and the killer instead starts to strangle her. She then kicks him off her but you can see he still has the knife in one of his hands as he falls backwards. (00:11:00)
Continuity mistake: In the final scene where Sidney gives the "coup de grace" to Billy shooting him in the head after he suddenly woke up, he holds the left arm above the mask on his top left, but when he dies, his arm is lowered and the knife appears from nowhere. (01:42:30)
Audio problem: At the party, Gale places a portable small camera on top of the TV to spy on the teens there. Later, the teens are watching the Halloween movie on this very TV and talking loudly. The movie is playing with sound when they are watching it. However, when we see Gale and Kenny's perspective as they are watching the feedback, they can hear the teens talking. However, despite the camera sitting right on top of the TV, it doesn't pick up the audio from the movie playingr. This mistake is repeated not long after when Ghostface is sneaking up behind the one teen left behind on the couch who's watching the movie at an even louder volume.
Audio problem: After Sidney and Billy have their fight in the hallway after he's released, the scene cuts to Mr. Himbry reprimanding two students wearing the Ghostface mask. He takes some scissors and starts to cut them up and waves the scissors around. The entire time, the scissors are making very loud, very distracting and unrealistic "SHING" sounds like metal swords sliding against each other in a hallway. These scissors simply would not make such obnoxious sounds with the simple movements he's doing with them. They even make the sound as he's cutting through the mask, which if anything would have muffled the sounds they made even more. (00:44:50)
Continuity mistake: After the girl's dad leaves the room, Billy holds up a red stuffed bear against the bed. In this shot, the bear is only half sticking up at about its arm level, but in the next cut, it's high enough its feet are visible on the bed. Then only a second later as Billy starts to stand up, a jump cut has his arms go from down to outstretched with the bear instantly as he starts to stand. (00:14:10)
Continuity mistake: After her first time with Billy, Sid brushes her hair. She puts the brush back on the bedstand, but the phone on it changes angle between shots. Later in the scene, you can also see the brush's handle sticking out from the side of the bedstand by the bed, or the one facing front. (01:18:55)
Revealing mistake: When Sidney discovers Tatum's body, we see her head intact, in particular the eye level. In her death scene, her cranium was being crushed right at the eyesocket. (01:07:25 - 01:22:15)
Continuity mistake: Mr. Himbry calls the two expelled students "Desensitized little sh*ts", pointing the open scissors at him. In the next shot he's looking at him but he changed position, since the scissors are gone from frame. (00:45:20)
Continuity mistake: Gale falls back on the unconscious Dewey after Billy kicks her. Her jacket is draped across the chest touching the white button of her vest. When Billy gets on his knees by her though, the jacket is fully open. (01:37:30)
Continuity mistake: Once Billy is done slashing his accomplice, he points the knife at him and tells him what to do; the knife hand keeps changing position between shots. (01:36:00)
Continuity mistake: Plummeting from the roof onto a boat, Sid causes a wetsuit (or a similar kind of fabric) to fall off. In the closer shot, the cloth is much more entangled than in the other view - look at the pink band. (01:21:55)
Continuity mistake: Off-balance on the roof as she shouts for help, Sid is hanging by the window with one hand or both depending on the shot. (01:21:45)
Answer: Characters in horror movies are notorious for not doing the most obvious things. It's most likely that she was scared and not thinking straight at receiving numerous threatening phone calls, particularly when she is threatened after hanging up on the killer.
Manky
Also, when she told the caller that she was "two seconds away from calling the police", his response was that they'd never make it in time. She also knew he was nearby as he made comments that indicated he could see her. She didn't know it was someone that knew her. She's a teenager and scared, so her judgement was clouded. She reached a point where she believed her only chance was to get away from him by either fighting him or getting out and hiding somewhere outside.
dewinela