Continuity mistake: In the very first scene, where Data is shuffling the cards, you can see his silver and gold chips are neatly stacked up straight. The camera then cuts to Commander Riker and then back to Data and his chips are crooked at the top and at the bottom.
Factual error: When the starboard nacelle causes the ship to spin, the view of the stars on the main screen on the bridge should move toward the right side of the screen, rather than standing still.
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.