This is what I can think of off the top of my head:
Andy Robinson was difficult to spot at first for me in Dirty Harry, due to the blonde hair, but once you see it, hard not to.
I wish Jolene Blalock looked the way she did in SG1, while on Enterprise.
David Warner, who has been in a ton of stuff can be hard to spot until you hear him speak, if you only know him from Star Trek 6 and as Gul Madred.
Trek actors in Stargate are easy to spot for me.
Robin Curtis wasn't easy to spot in The Gambit at first.
I was able to spot Conner Trinnier and John Billingsly in 24. Quinto, after seeing Star Trek 2009.
I remember seeing Marc Alaimo and Robert Picardo in the original Total Recall, and was easy to spot.
I've also seen Alaimo in reruns (after seeing DS9) in the incredible hulk, Knight rider, and Greatest American Hero.
John Delancie in Breaking bad (Jane's father) was a surprise once I realized it,
I remember how surprised I was when I heard the voices of Frakes, Sirtis, Dorn, Spiner, Burton and Mulgrew on Gargoyles, but knew them right away, except Spiner.
The reverse was true for:
Dwight Shultz: To me he STILL is Murdock!
I was surprised to see Bra’tac in Voyager episode Muse
Ronnie Cox in Deliverance, Stargate SG1 and Robocop is easy to spot (in fact, I knew him as Dick Jones).