Revealing mistake: In the shot where Geordie walks into the bridge of his mother's ship and finds that everyone is dead, if you look closely at the bottom right of the screen as Geordie is walking into the bridge you can see that one of the containers on the ground is quickly moved into place by a woman who is dead. (00:13:40)
Revealing mistake: The 'dead' woman moves the barrel towards herself. (00:13:40)
Revealing mistake: When Data gives Spot his food, when he puts the food on the ground you can see his normal skin (human skin). (00:16:35)
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.