Revealing mistake: At the beginning of the scene near the end of the movie with Lucius Malfoy fuming at Dumbledore in his office, Malfoy's hair is fanned back behind his shoulders. The lighting in the room illuminates the back of his neck, where you can see his real, short brown hair. (02:22:05)
Visible crew/equipment: In the dueling scene, when Snape pulls Malfoy back onto his feet, a cameraman is visible kneeling down on the far left of the screen. (01:07:46 - 01:20:00)
Visible crew/equipment: Rushing to make Hogwarts Express at the train station, the shot opens as the camera pans Ginny's trunk and two marks come into view. While Ginny and Mr. Weasley push her trolley, Mr. Weasley is cued and begins to say, "10:58, come on, come on!" just as the actor hits his tape mark on the ground below, while Percy's trolley hits its tape mark, so Harry is visible behind him, and cues Mr. Weasley to continue speaking. In the shot when Mrs. Weasley says, "Okay" for Ginny to go and in the shot as Harry says, "Let's go," their marks are gone. (Visible on DVD fullscreen.) (00:22:15)
Continuity mistake: When Errol drops the howler it lands in a bowl of potato chips/crisps. The seal to the envelope is open, but when Ron grabs it the seal is closed. (00:34:00)
Continuity mistake: When Myrtle is flooding the girls' bathroom, the tap that later doesn't work (the entrance to the Chamber) is happily churning out water - it's on the side nearest the door, so we see it clearly both times. (01:26:05)
Revealing mistake: Mrs. Weasley might as well knit like a muggle because she's not making any progress with magical knitting. In the Weasley's home and we see the magic knitting, the needle shoves into the hole, but no thread loops around and it is not advancing. The amount that has already been knitted is magic indeed since all the motion shown in the film produces no results. (00:11:25)
Continuity mistake: In the Quidditch scene, Harry breaks his right arm and cradles it on his chest, but when he is lying on the ground after he falls off his broom, he then leans directly on it when he sits up to get away from the Bludger. In the shot immediately afterward, he has his hand cradled in front of him again. (00:57:40)
Continuity mistake: When Lockhart falls down the hole into the Chamber of Secrets, we hear him hit the ground a second or two later. When Ron and Harry jump down, not only do they take longer to get down, they also slide down the pipe, rather than fall straight down. (01:59:40)
Visible crew/equipment: When in the Whomping Willow, Ron asks, "What's happening?" and Harry replies, "I don't know." After a few other shots, in the next two front close-ups of the boys as they scream, Harry, Ron and Hedwig are not alone in the car. A crew member's hand is quite visible holding the bottom of Hedwig's cage and in the 2nd close-up is actually laying the cage on its side. (Visible on DVD fullscreen.) (00:27:10)
Revealing mistake: In the many and lengthy dialogue scenes between Potter and Dumbledore, with countless facial close-ups, neither of them have glass in their spectacles. (02:19:40)
Revealing mistake: During the car flight (on the way to Hogwarts) Harry and Ron have gone out of control outside Hogwarts. During the scene when the car is approaching the whomping willow the car is massively out of proportion compared to when the car is in the tree. (00:25:25)
Visible crew/equipment: In Flourish and Blotts, when Lockhart says "Which, incidentally, is currently celebrating its 27th week atop the Daily Prophet bestseller list," the overhead camera pans up. As the camera passes his feet, Tom Felton's mark (two long strips of white tape) is perfectly visible on the balcony floor where he stands. (00:19:00)
Visible crew/equipment: After Harry climbs up the side of Salazar Slytherine's stone head in the Chamber of Secrets, the first time the Basilisk slams his face into the stone in the wide shot, the bottom of a crew member's denim pant leg and shoe are bizarrely visible lying on a work surface, beneath the stone head's beard. (Visible on fullscreen DVD.) (02:13:00)
Visible crew/equipment: While Harry is chased by the Bludger, Hermione says, "No! Even with a proper wand you could hit Harry..." Three shots later, as Harry passes the blue Ravenclaw tower, the back of his robe flies up and the metal base of the seat he sits on is perfectly visible. Some shots are filmed vertically, as he sits on the attached seat with the upright broomstick in front of him, but then edited with the background plate to look as if he is lying on the horizontal broomstick. Hence the need for a seat perpendicular to the broomstick. (00:55:00)
Revealing mistake: When Harry and Ron run at the wall to get to Platform 9 3/4 if you watch Hedwig's cage when they tumble over you can see a toy bird that is glued onto the perch. (00:22:50)
Revealing mistake: Harry and Ron's trunks are being thrown out of the car and in the shot just before Harry's close-up, Ron's trunk (the other trunk says HP) slams down on the grass. The top of this trunk has a large perfectly symmetrical hole in the center with four round marks around it, which was probably involved in the throwing somehow. In the wide shot as Hedwig is being tossed out, the trunks are clearly visible and there is no hole in either, nor is there a hole in Ron's trunk at any other time. (00:28:10)
Revealing mistake: When Harry is fighting the Basilisk on top of the skull in the Chamber of Secrets, you can see that there is a safety tip on the end of the sword. (02:12:55)
Continuity mistake: When Ron and Harry are escaping from the spiders in the flying car the passenger's side window that broke earlier hitting the Whomping Willow is not broken, you can see Ron's reflection. A minute later you see that the window is broken again. (01:48:40)
Continuity mistake: There is a scene where Harry is talking to Ginny in the Chamber, and he is holding his arm right below the wound. In this shot Harry has blood on his hand. When Ginny notices the wound, Harry moves his hand up to cover it. Now his hand is spotless. In the very next shot, however, Harry has blood on his hand again. (02:16:30)
Visible crew/equipment: After the car spits them out and drives itself away and they are chasing after it, if you watch closely you can see the shadow of someone driving the car. (00:28:30)
Answer: It was traveling through the pipes that it could fit into.
So how was it able to get into the girls restroom and kill Moaning Myrtle many years ago? All of those pipes in the restroom aren't large enough.
The girls' bathroom is the entrance to the chamber of secrets. She was in there when Tom Riddle opened it and let the snake out.