Continuity mistake: When the security man gets tossed by the Borg, as he lands and rolls next to the far wall, his phaser falls out of the holster, and lands next to his shin. A second later, it's about two feet away from his leg.
Continuity mistake: Wesley says that he's departing shuttle bay 2, but on the floor, it says shuttle bay 3.
Continuity mistake: As La Forge prepares to transport the Pakled shoots him. He reacts, and a second later reacts again.
Continuity mistake: The Bringloidi leader keeps saying that they've spent 300 years building their society, but this episode takes place in 2365, and it's stated that his ship launched in 2123, making it 200 years.
Continuity mistake: In the beginning of the episode, Riker, Geordi, Worf, Data, and Pulaski are playing poker. Pulaski puts two neat stacks of chips in the center beside some chips in a pile. It then shows Worf putting similar stacks in the center but Pulaski's stacks are not visible and they should be. Then when Worf wins the hand, the camera is showing a wide shot of him pulling the chips towards him and you can see his and Pulaski's neat stacks of chips.
Continuity mistake: When Data is dealing the cards for 7-card stud, they show him dealing 1 card to everyone. In the close up, he's now dealing everyone a 2nd card. When it cuts to his hands however, everyone only has 1 card and he's dealing them a 2nd card again.
Continuity mistake: Near the end, when Pulaski finally stops the infection in Riker and is just about to contact Picard with her success, look at Riker lying on the table. His eyes are open, then he closes them slowly. The next scene has Riker opening his eyes for the first time after recovering from the procedure.
Continuity mistake: After Pulaski examines Riker's leg, and readies to beam up, he leans to the right to get up. In the wide shot, he does it again.
Continuity mistake: In the opening sequence, the ship is positioned so that Shuttle Bay Two is aimed at the phenomena. But a moment later, the ship is facing into the stellar material.
Continuity mistake: When Geordi falls down the hole in the beginning of the episode his visor is seen to his right. When he wakes up he finds it to his left.
Continuity mistake: After Wesley makes his neutrino pulse suggestion, Picard tells him to go do it. Wesley walks off camera. Then there's a commercial break, and Wesley is back on the bridge in the long shots. A few minutes later we hear the turbo-lift and Wesley walks back onto the bridge.
Continuity mistake: When talking to Ral, Riker takes a sip of his drink, sets the glass down, and clasps his hands. A second later, he repeats clasping his hands.
Continuity mistake: When Data goes into his laughing fit, Troi is at her regular seat, but when he stops and remarks it was a wonderful feeling, the wide shot of the crew shows her seat empty. But then, when Picard is hailed, when the moon returns to its orbit, Troi is back at her seat.
Continuity mistake: As Q approaches Picard and Riker's position while playing a trumpet, both of Q's hands are on the horn. A second later, his left hand is down by his side. Then, after he makes cigars appear in Picard and Riker's mouths, his left hand is extended as he says the first part of his lines. A second later, both hands are on his horn.
Yesterday's Enterprise - S3-E15
Continuity mistake: In the episode 'Yesterday's Enterprise' (season three, episode 15), watch the final scene with Geordi and Guinan in Ten-Forward. Although time and the universe has returned to normal, Geordi is still wearing the uniform from the alternate timeline (note the sleeve detail, and the extended neck piece).
Continuity mistake: While Tam is talking to Deanna and sits down, he says "You understood... at least a little, how I felt." While saying this, he places his left hand on his right leg. While still saying that same line, it cuts to an angle from beside Deanna and his left hand is suddenly between his legs.
Continuity mistake: To view their orders and mission, Data puts a drive into the Enterprise's computer while standing with Riker and Geordi. Riker is seen standing but leaning casually away from Data on Data's right, and Geordi is standing off to Data's left. It then cuts show the computer screen, and the three men are reflected in it. The reflection shows that Riker is suddenly almost leaning over Data's shoulder, and Geordi has moved to standing behind Data's left shoulder.
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.