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).
Factual error: The second piece of music played at the concert is by Brahms and not Mozart.
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.
Visible crew/equipment: When Riker first beams down he uses a flashlight - if you look closely you can see the electrical cord that runs down his sleeve to an off-screen generator.
The Best of Both Worlds (1) - S3-E26
Revealing mistake: During the phaser battle on the Borg ship, a detonation wire is seen attached to the right foot of the drone that Beverly kills.
Who Watches The Watchers - S3-E4
Visible crew/equipment: Just after Nuria says "your power is truly boundless" a few seconds later a microphone can be seen dropping and quickly being raised again at the top of the window.
Who Watches The Watchers - S3-E4
Revealing mistake: About 38 minutes into the show, when Dr. Mary Warren dies and there's a close up of Picard & Nuria, you can see the her hand moving up and down as she breathes.
Yesterday's Enterprise - S3-E15
Visible crew/equipment: During the scan of the temporal rift at the beginning of the episode, the tip of the microphone briefly comes into view at the top of the screen above Captain Picard's head when Data says, "Nothing I have seen before." [Not visible in the remastered Blu-ray version].
Continuity mistake: At the end of Barclay's sword fight on the holodeck, he has the hologram of captain Picard against a wall. When the angle is on the hologram, Barclay's sword is pressed into his chest. However, when the angle is on Barclay, the sword is several inches away from the hologram.
Other mistake: When Picard walks into the brig to talk to Q, something falls in the doorway and Picard almost kicks it with his foot. The door then closes on the item that fell. (00:11:20)
Other mistake: As Troi describes her idea of "real" chocolate to the computer, it interrupts her with a warning about the nutritional value of her request. The reply is in two parts, and between the parts, you hear the intake of breath, something no computer would do.
Other mistake: While Riker looks over Data's shoulder, the make up on Data's eyebrows is incomplete. The left is lighter, the right is darker.
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: 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.
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.
The Best of Both Worlds (1) - S3-E26
Other mistake: When O'Brien activates the transporter, only the very bottom indicator lights come on, even as he moves his hand up. Usually the indicator lights match the movement of the hand.
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.