Photo of Walter that Will picks up from Walters house remains dry despite the fact that Will fell into the water when chasing across the logs. There are no signs of the fact that it got wet when it is next shown.
When Ellie Burr asks Dormer to sign the report and Dormer tells her to be sure of all of her facts, he walks out of shot having taken his glasses off. In the next brief shot of him passing by so we can see Ellie's reaction, his glasses are back on his face.
When Dormer first enters Finch's apartment, he picks the lock using a credit card. After the door is open, we see him getting his hand back to his pocket and in the next shot he has some snacks in his hand to bribe the dogs to let him in. Since his hand motion is very quick and brief, it either serves to get the snacks from the pocket, in which case the credit card simply disappears, or to get the card back to the pocket, in which case the snacks materialize out of nowhere.
In the initial shots of the Beech 18 seaplane, when supposedly cruising to destination, the aircraft is clearly flying too low and has one notch of flaps down. This is not a normal cruise flight configuration.
When the police officers are waiting for the murderer to return to the cabin for the girl's backpack (Scene 8 on the DVD), they are talking about firearms. One of the Alaska cops says "Yeah that's nice, mine's all plastic save the barrel and firing pin - never rusts. What do you carry down in LA?" In the real world, the only part on his Glock that is plastic is the frame. The barrel, firing pin, trigger components, and SLIDE are all metal. Seeing how the Glock's frame (that's the big part on top) is 100% blued metal, it rusts just as easily as anything else.
The information regarding Glock pistols is incorrect. The slide, while it is steel, does not have a blued finish. The finish is called Tenifer, and is proprietary to Glock. The Tenifer makes the slide essentially rust proof. The only parts that may have a tendency to rust are the sights which have a different finish. Gun enthusiasts often refer to the Glock as "dishwasher safe".
On the ferry, the Pacino character and the Williams character have a long discussion. Outside the window of the ferry the foreshore scenery does not move, yet they are supposed to be sailing somewhere.
The comment about Walter's trigger finger position is partially correct. A double barrel shotgun usually has two triggers, one behind the other and one for each barrel. It's reasonable that an open trigger would be visible while firing with the second trigger.
The goof items below may give away important plot points.
When Walter falls into the water though the bottom of the boat house at the end of the movie he falls about 1 foot before he hits the water. In the next scene when Detective Ellie is leaving the boat house the water is 5 or 6 feet below the bottom of the boat house.