For dopers from the UK, perhaps there was that moment when you realised that the headmaster from Grange Hill was the same admiral force-choked to death by Vader in ESB.
And if you want a really stupid answer, I had to read a film review to realise that Julie Christie played different characters in the same film, Fahrenheit 451 :smack:
NO WAY!! That’s hilarious. I did not know that.
An embarassing one for me is Tim McInnerny. He played Lord Percy in the first two seasons of Blackadder. Then in the fourth season of the same series, he played Major Darling and his character was so different it took me several episodes to realize if was the same actor.
He’s also Veronica Mars’ father.
Don’t forget Pat Morita, Ron Howard, John Ritter, Leslie Nielsen, and George Wendt. Tons of others, too.
I always get a laugh when I see an actor playing either a Chinese or Japanese character and I recognize them from having played a Korean character on MASH.
Oh yeah - one more - I still can’t look at Rene Auberjonois’ face on Boston Legal without seeing it in my mind with the makeup from Deep Space Nine.
The Muslim terrorist kid in the early episodes of the last season of 24 was Van Wilder’s Indian sidekick.
He was also the nutcase Lt. Coffey in The Abyss
And Vincent D’Onofrio is the alien in the brand-new Edgar suit in Men in Black 
Not to mention the exceedingly prim and annoying Clayton Endicott III from Benson.
I haven’t been able to see the lovable Randy on *My Name is Earl * in the same light since I realized he was the fat racist fella in American History X
Aw, man! I was just getting my lust on for Sir Mark Brydon in The State Within and NOW you tell me that he’s Lucius Malfoy?!? AAAaargh!
Recently it clicked for me that D’Onofrio played the bug-alien in the “egger suit” in Men in Black. I was watching the film for the dozenth time and it just clicked: Hey that’s Detective Goren! He is an impressive actor.
Also the bad guy in Jennifer Lopez’s The Cell
Not to mention the original Father Mulcahy in the movie MASH*
Don’t feel too bad, there are still people who don’t know that Michael J. Fox played every speaking role in Back to the Future Part II, including the little boy you thought was Elijah Wood. He’s amazing.
The guy who played The Cowboy in Innerspace is The Doctor from Star Trek: Voyager - Robert Picardo
Making those scenes where his mom was flirting with him just that much creepier.
I was surprised to see how many weird roles Picardo has had over the years before I noticed him. He was the theater owner in Matinee, the voice of Johnny Cab (and the model for the maannequin) in Total Recall, and he’s totally unrecognizable under makeup as Meg Mucklebones in Legend.
I couldn’t believe that Mandy Patinkin played both Inigo Montoya (“Choo killed my father. Prepare to die”) and George Francisco, the Newcomer cop in Alien Nation (the film version).
My “OH SHIT” moment was in the late 90’s when I finally realized why Morgan Freeman always looked so familiar to me.
When growing up my favorite show was The Electric Company on PBS which he was a regular cast member of for years.
Shhhh-
-it.
SHIT! That’s the same dude!