I just realized the other day that during the ‘busting’ montage, you see both Venkman and Egon wearing the Ecto googles. Never noticed that before.
I checked that out and when I did I noticed the hat thing ray is wearing when he accepts the chicken from the oriental gentleman.
Did you notice the Say Puft Marshmellow ad on the bldg across from the station.
Also during the Busting Montage, when Casey Casem is talking and they are running down the sidewalk, Venkman has his leg hose wrapped around the trap handle and right before they cut back to dana barretts apt, you can see it unravel
When the Ecto busts out of the firehouse for the first time, the sniffer tower is bent down like it was during the deleted bursting ticket scene in the montage
It’s obvious to me why his shock mount was pointing up, it’s because his proton pack was turned on.
I noticed a few years ago (and many others have probably also noticed) that the newspaper photograph in the scene where the guys geceive the ducks is actually of them crossing the streams.
When Dana opens the fridge, they’re a big, very noticable bulb in Zuul’s mouth. Darn HD… 
During Dana’s interview, there’s a couple of throwers on the table behind her.
I never noticed that the cop that tells them the mayor wants to see them in the first movie is the same actor who played Carl Winslow. I’ve seen this movie too many times to count and I just noticed it today.
The Carl Winslow thing is very similar to how I never noticed Ben Stein in Ghostbusters 2 (which ended with Hardemeyer being fired I believe)
Back about a year or so ago, I finally noticed that the montage scene with Ecto-1 is making a left turn is actually flipped. I noticed because of the logo on the side of the door is backwards and the gear on the roof rack is on the wrong side. lol
At the end of GB1, Ray unplugs his leg hose as he gets in Ecto-1 and the doorman shakes his hand.
When I first watched GB1 and watched it over and over, I never noticed when Peter is telling the manager at the hotel about the costs that Egon was holding up his fingers to clue Peter on what to charge.
After I noticed that, it made me wonder who really was the capitalist on that one.
This one is easy to miss depending on what version of the movie you own (dvd, off the tv or Blu-ray etc). In the blu-ray version I noticed clearly for the first time the stunt doubles used when Gozer zaps them after “Then … DIE!” Also, due to the bigger/wider shot, you can see a large (what looks like) a metal pole being dragged into the smoke at the back, probably dragging the guys back.
In addition the next shot of them heading for the edge of the building, you can see winstons cable and what looks like a treadmil as a piece of debris moves and pings off somewhere.
HD is amazing but pointing out all the props, gizmos and stunt doubles due to a clearer/sharper picture isn’t so great lol.
One thing I noticed while watching GB1 on the big screen was in Louis’ apartment, during the party scene, there is a pretty funny picture of him in a cap and gown holding a diploma. Not sure if its high school graduation or his graduation from Law School 
I noticed that recently too, and I couldn’t quite fathom the fact that our favorite genius scientist was also a little greedy. 
Probably not greed, just that unlicensed nuclear accelerator’s are expensive to build and maintain =)
Not to mention all the other gears’ upkeep, paying the mortgage on Ray’s house, the power, the car maintenance, paying on the Firehouse, Janine’s salary, hordes of other bills…
And yeah, I just noticed that now myself! Egon, you capitalist whore! And you know we love you for it! XD
When the boys are running screaming from the library, there’s a curly-haired extra standing on the library steps, looking at the commotion.
As they exit the bank and walk down the street, talking about Ray’s mortgage, the same extra is stalking them.