Revealing mistake: When the force field is activated in the Jeffries Tube, we only see the flash of light from off-screen. The effects team neglected to add the force field effect.
Revealing mistake: As Riker is being dragged toward Armus, the wire pulling him can be seen disturbing the sand and the surface of the slime pool.
Visible crew/equipment: When Lt. Chang tells the candidates, "It's unfair that only one candidate from Relva will attend the Academy this year," there are two tape marks visible on the floor between Chang and Mordock. (00:49:00)
Visible crew/equipment: When Will is wearing the (unforgettable) indigenous apparel for his meeting with Beata, the moving boom mic casts a shadow on the wall on the right side of the screen. (00:16:25)
Plot hole: During evacuation, when Wesley is beamed off the ship, there are room for one more person on the transportation pad. You would think they would like to transport as many people away as fast as possible.
Other mistake: As Picard and Robert walk alongside the large bush just before their scuffle, two outgrowth branches on the bush get snagged onto the camera and can be seen snapping back into place.
Visible crew/equipment: Two white lights reflect off Armus' pool when Picard walks up to it. Since the planet is devoid of all other life, this can only be a reflection of stage lights.
Other mistake: As Beverly points to a planet on the monitor, the read out on the planet is upside down.
All Good Things... (1) - S7-E25
Character mistake: When Picard jumps to the past, he meets Data for the first time in engineering. If you look at Data's uniform, he is wearing the rank insignia of a Lt, Junior Grade.
Character mistake: Captain Picard wonders why the boy would want to go back to his captors if they abuse him and Dr. Crusher tells him it's not uncommon, it's called the Stockholm Syndrome as if Picard doesn't know or understand. However, in S03E12, "The High Ground", Dr. Crusher is held captive and when Picard is talking to her (after he is captured trying to rescue her), she begins to appear sympathetic to her captors and Picard says "I don't have to remind you of the psychological impact of being a hostage." Picard would be aware of why the boy may wish to return.
All Good Things... (1) - S7-E25
Other mistake: When Picard first meets Data in the past, Data's rank insignia is wrong. He's always held the rank of Lieutenant Commander, signified by two gold pips, and one with black in the center. However, Data has one gold, one with black, signifying Lieutenant.
Revealing mistake: The 'dead' woman moves the barrel towards herself. (00:13:40)
Encounter at Farpoint (1) - S1-E1
Other mistake: As the Enterprise is fleeing from Q, it peaks around Warp 9.6 which according to the fact books, translates to around 1000c in sidereal space. Also according to the fact books, the saucer section is not capable of sustained Warp flight on its own. Also, at the beginning of The Chase scene, the Enterprise is on course to Farpoint, Q's fence is perpendicular to it. The Enterprise does at least a 90°+ turn beginning her escape maneuver prior to separation, so it is definitely not flying past Farpoint during The Chase. Still the saucer section somehow makes it to Farpoint a mere 51 minutes after the engine section. There's no evidence at all in the dialog that it was Q's doing.
Who Watches The Watchers - S3-E4
Visible crew/equipment: Just after Nuria says "your power is truly boundless" a few seconds later a microphone can be seen dropping and quickly being raised again at the top of the window.
The Royale - S2-E12
Continuity mistake: As the Texan follows the dice on Data's last throw, his glasses are on. As he complains about losing his bet, his glasses are suddenly missing.
Plot hole: The entire premise of the episode is contradictory; the franchise makes abundantly clear through the so called Prime Directive that the Federation abides to a code (often creating moral dilemmas that may require to stretch the rules) that says that their staff is not supposed to interfere in world that haven't reached warp capabilities, nor involve themselves in their internal matters. In the first half of the episode, Riker and the others just go 'mingle' with the frisky natives without a care in the world, and yet in the second half the Prime Directive itself is referenced explicitly and it is part of the plot.
Suggested correction: They are just enjoying the hospitality of the planet's inhabitants. They are not interfering in their internal affairs or the development of their species. It's only when Wesley gets into trouble that the Prime Directive comes into play. The entire mission of The Enterprise is to make contact with other planets.
Even though they are supposed to make contact with other planets, it's pretty clear in the rest of the series after the first few episodes of Season 1 that they do not (intentionally) contact people that have no warp capability.
Plot hole: The binars needed two people to upload the data back to their world but they only intended on keeping Riker in the holodeck. Riker is just one person. Picard came into the holodeck purely by accident - even Minuet said this was true. If Picard hadn't been there, Riker could not have uploaded the data on his own and the binars' plan would have failed.
Suggested correction: The Binars only trapped Riker because they thought they might need someone to restore the computer for them, which wouldn't have taken 2 Enterprise crew members. It was the fact that the Binars got to their planet too late that required Picard and Riker to work together. However, the Binars already knew Picard was on the Holodeck before they initiated their plan because when Picard exits his ready room he says he's going to join Riker. Had Picard not done this, certainly the Binars could have kept a 2nd crew member on-board somehow, if needed.