Just before Stacey is killed we see the entire chat log. The lines, "Too bad I have to kill you now" and (most importantly) the three lines of "I did it for the lulz" is gone as well.
In the opening scene when the babysitter is on chat: "I did it for the lolz" message is typed and is seen on her screen as sent by "You" as if she typed it. But then a few seconds later, we see the screen again and the message was sent by the guy who she's chatting with and then she gets killed.
Right after the scene where Ashley and Proxy pretend to make out when they decide to test out the myth of Smiley, you can see the full chat log on the screen while they still have to type it in the next scene.
When Ashley is walking through campus after having a drink at the second party she's wearing very light blue jeans, but after she thinks she sees Smiley and starts to run, for one scene her pants are clearly a different pair of dark blue jeans.
When the detectives are talking to Ashley and they reveal the Youtube video where she's smashing her computer in the library, you can clearly see the editing options on top of the video (edit video details, edit annotations, audio swap, etc.). This would indicate that the detectives were the ones who uploaded the video, as they were logged into the account where the video was uploaded.
When Zane threatens Ashely with a gun, he pulls back the slide to cock the gun before pointing it at her. When he does, the slide can be seen locked back before the edit, showing the gun is unloaded
When Ashley and Proxy are testing the Smiley myth by visiting a video chat website, the camera on the website shows that the girls are not talking even though we hear them speaking in the room.
After the conversation about Smiley and the medication at Zane's house, you can see a crew member out the door and in the reflection of another.
Around 48:18.