Spike's chip first activates when he tries to attack Willow, which we later learn is caused when he tries to hurt a human. If this is the case, it should have activated when he escaped from the Initiative and attacked the two scientists.
At 4:03, as Giles raises his drawing to show it to Xander, he knocks a bit of crumpled paper from the coffee table onto the carpet. Also, he is resting his head against his open hand. When the view cuts back to Giles after Xander's brief comment, the bit of paper has disappeared and Giles is resting his head against his closed fist.
When Spike and the other vamp are running down the Initiative hallway to escape, you can briefly see Spike's reflection in one of the cell doors.
At 26:35 the insert shot that is supposedly of an elevator going down, as seen from above, is in fact a shot of an elevator going up, as seen from below.
At the end of the teaser, as the camera pans to reveal the seemingly endless row of cells in the Initiative, if you look closely you can see there is a large mirror three cells down from Spike's that is helping produce that illusion. As the pan continues, you can also pick out the reflection of the duplicate mirror behind the camera that completes the illusion. And finally, just before the scene cuts to black, a reflection of the camera equipment filming the shot starts coming into view at the right-hand edge of the picture.
In the teaser, as Spike wakes up in the Initiative and the camera pulls back, his red sweater is hanging open and you can see the imprint of what looks like a microphone cord concealed under his black T-shirt. Then, when the view cuts to a different angle, suddenly his sweater is closed.
Although his chip is supposedly operative since before his escape from the Initiative, Spike manages to harm several humans between then and the end of the show. During his escape and again during the fight in the dorm hallway, Spike chokes people, throws them into walls, and hits one in the head with a fire extinguisher, none of which appears to activate his chip. The only time it's shown to work at all is when he attempts to bite someone, which distinctly contrasts with the chip's functioning in all subsequent shows.
Whilst Spike is being held in his cell by the Initiative a blood pack is dropped into the cell from the roof which Spike it told by a neighboring vampire is drugged blood. Spike is later seen laying on the floor having faked drinking the blood with the drained blood pack next to him. What has he done with the blood? Shots of the cells clearly show that there are no drainage holes in the cell floors and as all the cells are monitored by CCTV, as seen in later in the series, any attempt to do other than drink the blood would have been monitored.
Spike and Willow are talking in Buffy's dorm room after Spike proves unable to bite Willow. Spike says he's 126 years old. 1999 (the air date) minus 126 is only 1873. This cannot be Spike's years as a vampire as we learn that he was turned in 1880. So he has been a vamp for at least 119 years, granted. However that would only make William seven years old at the time of his death. Which cannot be as he is in his upper twenties to at least his mid-thirties eternally.