Other mistake: At the beginning of the episode, when Picard and Riker leave the bridge to head to the teleport deck to receive the admiral and his wife, as the lift door opens Patrick Stewart makes a face at the film crew off-screen as he walks off the bridge. (00:02:05)
Visible crew/equipment: When Yar and Worf prepare to cut through the wall beneath the capital city, you can see the flash-powder cord before they shoot and the phaser beams get F/X'd in.
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