Continuity mistake: When Data and Troi are scanning the list of elements, searching for something to cause an explosion, the list is scrolling "up" the screen. When she tells Data to stop and go back, he does, but the list continues to scroll "up" instead of back "down".
Devil's Due - S4-E13
Continuity mistake: As the camera does a close up on the shuttle craft's forward window as it approaches the Enterprise, the dorsal section is seen getting larger the closer they get. As Data and Picard talk, the image fades out, and a minute later, the angle changes, the dorsal section is again seen getting bigger, and the ship disappears.
Continuity mistake: At the end, Picard is looking out the window when Worf comes in. There is a plant next to Picard in the close up, but it's missing in the long shots.
Continuity mistake: In the opening sequence, the venetian blinds in Dixon Hill's office rearrange themselves between takes. They're intact after the shooting, but moments later, despite being untouched by anyone in the room, they're broken and have a huge gap in them. (00:07:00)
Continuity mistake: When commander Data is reciting the code to lock the rest of the crew out of the computer, he states the code '1-7-3-4-6-7-3-2-1-4-7-6-C-3-2-7-8-9-7-7-7-6-4-3-T-7-3-2-V-7-3-1-1-7-8-8-8-7-3-2-4-7-6-7-8-9-7-6-4-3-7-6-LOCK', but when the computer screen is shown the code reads '1-7-3-4-6-7-2-1-4-7-6-C-3-2-7-8-9-7-7-7-6-3-T-7-3-2-V-7-3-1-1-7-1-8-8-8-7-3-2-4-7-6-7-8-9-7-6-4-3-7-6-LOCK.'
Data's Day - S4-E11
Continuity mistake: At the end, Picard shows Data a baby born during the crisis. Neither of them touches it, but between shots, the baby's blanket changes positions. First it's almost completely covering his head and face, then it's pulled down and showing much more of him. (00:42:15)
Devil's Due - S4-E13
Continuity mistake: In Picard's quarters, Ardra appears as Troi. After reappearing as herself, she puts her hands on her hips. After the angle change, she repeats the motion.
Continuity mistake: In Dixon Hill's outer office, Gloria's right arm goes from being up to by her side and back between camera cuts.
Continuity mistake: When Riker approaches his "son" he raises his right hand to calm him. When the camera angle changes he is suddenly raising his left hand.
Nth Degree - S4-E19
Continuity mistake: When Barclay enters the Holodeck to create an interface, he calls for a chair, an input for the left and a display for the right. When Picard comes to question him, the input and display units are gone.
Answer: Yes. The doors to the holodecks can be locked when in use by anyone aboard the Enterprise so they couldn't be disturbed. However, high ranking officers like Captain Picard could override the doors as it's seen that overrides are in use even for the crews quarters. Even Barclay, when he's indulging in one of his fantasies could have the doors to the holodeck unlocked by an override code.