I noticed the pants thing when I first watched it, but didn't notice it was Walt's pants from the Pilot until someone mentioned it over on the IMDB boards. It's also amusing to see Walt rolling the barrel after Hank laughed at the video in 2x01 of Walt and Jesse trying to carry the barrel they stole on the S1 finale instead of just rolling it. "It's a barrel, dummies. It ROLLS." (He didn't know it was them at the time, of course.)
Here's another thing that somebody on another forum caught:
The way Walt falls to his knees in slow-mo when Hank is killed and the way Gus in 4x08 falls to the ground in the flashback when his partner was killed by Hector, and the subsequent shot above, are both incredibly similar. And we all know that Gus eventually got his payback on Hector and the Cartel (actually, Hector got the last word in the end with the wheelchair bomb, but that was thanks to Walt. Gus still took out Don Eladio and the majority of his people.) Not that there should have even been a question about it at this point, but he know who that M60 is for.