Goofs... 

Charlie's room features the Lego set 6190 Shark's Crystal Cave, which was released in 1996 whereas Charlie died the year prior.

Goofs... 

In the beginning three minutes of the episode, when the crystal O'Neill reaches down to pick up the real O'Neill's weapon, his boots are not fully laced and when he emerges from the gate in the next shot they are laced to the top.

Goofs...

When SG-1 returns from the planet and are in the Briefing Room with Hammond talking about the crystals, they zoom in on the fake O'Neill and you can see Teal'c and Hammond behind him turn and look at each other and their lips move like they are talking to each other but Dr. Daniel Jackson is the one speaking.

Goofs...

When the real Colonel Jack O'Neill dials back to Earth, Major General George S. Hammond informs Captain Samantha Carter whoever is dialing in used a GDO to deactivate the iris. A wormhole has not been established at this point making it impossible for someone to send a deactivation code.