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.
Revealing mistake: In an overhead shot of the Brattain, it's clear that the ship's (misspelled) name is just a poorly applied decal.
Visible crew/equipment: While Picard talks to Rasmussen in his ready room, when Rasmussen says, "I can't get quite as worked up as you," it cuts to a wide shot, and there's a white tape mark (T-mark) taped to the floor under the desk. (00:32:45)
Visible crew/equipment: On Romulus, when Picard and Data are told, "Eat your soup, courtesy of a loyal establishment," the camera turns to face the duo dressed as Romulans, and an actor's T-mark is visible on the ground behind Data. (00:42:40)
Visible crew/equipment: Boom action is visible in reflections on computer consoles in the back, while Picard and LaForge talk to each other on the bridge. (00:40:15 - 00:40:46)
Continuity mistake: When Data goes into his laughing fit, Troi is at her regular seat, but when he stops and remarks it was a wonderful feeling, the wide shot of the crew shows her seat empty. But then, when Picard is hailed, when the moon returns to its orbit, Troi is back at her seat.
Continuity mistake: When Lore backhands Worf in the turbolift, his eyes close but open back up as he is sinking down. Then, in the close-up, his eyes are closed again.
Visible crew/equipment: After the intro, when Garvin tells Gia to go home, Data is standing on the stairs and the actor's green mark is somewhat visible at Data's feet. This green T-mark is clearer when Data hands Garvin the radioactive box he's carrying. Then, when Garvin walks over to the table, there's an orange T-mark on the ground. (00:04:55 - 00:07:40)
Visible crew/equipment: After Gia tells Data to remove his hood, he asks how long it will be until Skoran and the others return. Then, when Data tells her that he must hurry he takes a step forward, and a green tape mark is visible on the floor. (00:36:35)
Visible crew/equipment: While Troi tells Riker about the Captain's grueling negotiations between the Gemarians and Dachlyds, when Picard joins them on the bridge, there are three visible T-marks taped to the floor. (00:01:50)
Other mistake: During his final discussion with Tomalok, Picard tells him that the Enterprise will escort the Romulan ship back to The Neutral Zone. However, as the episode ends, the two ships are heading off in opposite directions. Both the Enterprise and Romulan warbirds have demonstrated in the past that they can spin on their axis from a standstill. There is no reason for either ship to head away from the other to turn around.
Answer: He brought the Borg to the Alpha Quadrant and showed them that it was full of worlds waiting to be assimilated. Guinan's homeworld was their first stop, and they assimilated everyone and took over the planet, leaving The Survivors of her race without a home. Q is ultimately responsible for that.
Captain Defenestrator
By the time Q takes the Enterprise to meet the Borg, Guinan already knew who they were and they had already destroyed her world. Therefore the above answer can not be right. I believe Guinan is much more than she appears, and her people have had encounters with the Q in the past. It is these interactions, that obviously were not pleasant, that fuels her distrust.
oldbaldyone
That's what the above answer is saying. Q brought the Borg to the Alpha Quadrant (not Earth) and the Borg destroyed Guinan's home world in the late 2200's, which is why she hates Q. Although she met Q in 2160 and they both saw each other as enemies right away.
Bishop73