Revealing mistake: As we know, the watery image of the event horizon is projected onto a blue backdrop. In "Origin" the third episode of season nine, Ger'ak comes through the Stargate and we can see the image of the event horizon, though faint, around and above the Stargate itself and is not confined to normal area for the event horizon. Blatantly visible if you look for it on the airing of the episode on the Sci-Fi network.
Revealing mistake: When the village canon touches the hot poker to Teal'c's forehead tattoo, the profile view shows that the edge of the applique used by the makeup artist has separated from Chris Judge's face. It apparently was not glued down securely enough, and this is not an isolated incident. It happens quite frequently throughout the entire series.
Revealing mistake: When Teal'c and O'Neill are passing by Jupiter (after the failed missile burn), you can see a reflection in the background window of the cockpit. But the reflection is not the backside of their helmets, but instead is the front sides matted on to give the minor illusion of a reflection.
Revealing mistake: When Carter says "It will just take a few minutes to initialize" at the super-gate and presses the screen on her hand you can see for a brief second that it is a video recording since the options for play, pause and stop show in the options bar on the screen.
Revealing mistake: In the scene when the timer exceeds 38 minutes, said timer is displayed in big digits on a computer screen. There is a shot of General Hammond with the Stargate in the background. There is a pane of glass between the General and the Stargate. In the shot, General Hammond turns and looks at the gate, and then the camera's focus changes from the General to the Stargate itself. In doing so, a reflection of the timer in the pane of glass is also brought into focus. However the timer is displayed correctly when it should be flipped left to right as all text is when it is reflected in either a pane of glass or a mirror.