Continuity mistake: After drinking some spiked Champagne from Lore, Data falls backward, yet lands on his face.
Continuity mistake: Near the end of the episode, right after Lore fires the phaser at Dr. Crusher, Data tackles Lore and knocks the phaser out of Lore's hand. The phaser flies towards the cargo transporter and when it impacts, it breaks in two. You can see a piece of it go flying off to the left. Then shortly after when Data throws Lore onto the transporter pad and Lore picks up the phaser, it's in one piece again.
Continuity mistake: As Lore opens a channel, there's a blinking red light over the key he uses that disappears after the angle change.
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.