I couldn’t pick most of today’s Hollywood celebrities out of a police lineup. Attractiveness has become so standardized that I can’t tell one “pretty” face from another. Today’s famous faces have almost no distinguishing character traits at all. It’s boring as plastic fuck.
I have seen pictures of Hedy Lamarr and thought she was Vivien Leigh.
IIRC a couple of times I was not sure if a movie I knew about or had seen starred Ben Stiller or Adam Sandler. E.g. “Meet the Parents.”
I once misremembered the nationality of an actor. I thought Colin Farrel was Australian and was surprised to learn that he was Irish. I think I mixed him up with Mel Gibson who holds Australian citizenship. Farrel I think looks a bit like a younger Mel Gibson (certainly the thick dark brown hair).
I’m pretty sure there were other examples. I might remember them later.
As a young person Anthony Michael Hall had a distinctive look. As an adult he looks exactly like Terry Serpico.
You certainly did if you thought it was Cary Grant instead of Clark Gable in The Misfits.
Indeed. Those two looked so much alike, they could have been sisters.
I think the second one was titled Mr Novak. In MAD Magazine, it was called “Mr Nudnik.”
Are you sure it was James Franciscus and not Richard Chamberlain in that one? 
There is some resemblance. But their different hair colors made it easy for me to tell them apart.
There are different reasons for confusing one person with another. Like Too_Many_Cats2, I can’t tell many younger actors apart. I don’t know how much of that is increasingly homogenous standards of attractiveness, or my age.
For years, I thought it was Arthur Hill (aka “Owen Marshall, Attorney at Law”) who played the Nazi that spoke American English opposite James Garner in 36 Hours. Turns out it was Rod “The Time Machine” Taylor instead.
In some of their roles, I could easily confuse Joe Pesci and Al Pacino.
Amen to that! 
Are these non-speaking roles? I’ve never heard anyone whose voice could be mistaken for Pesci’s.
I was thinking in particular about the courtroom scenes in Scent of a Woman and JFK.
Yeah, Pacino was more animated than usual in SoaW.
Can I add Stuart Margolin and John Saxon?
Not physically similar, but it always seemed that one (both?) was a “guest star” on the Barnaby Quincy Cannon McMillan Rockford juggernaut.
Sara Paxton and Sarah Polley.
Not by appearance (or accent), but I have to pause and remember which is which as far as William Hurt, John Hurt, and John Heard.
Speaking of Star Trek, Armin Shimerman and René Auberjonois confuse me because they both played sort of Ayn Rand types in classic video games (although Robert House is a whole lot of Howard Hughes).
I think when I was a kid watching Home Alone I mixed up Joe Pesci with Danny DeVito.
Not helped by the fact that the SAG (Screen Actors Guild) is supposed to help prevent that kind of confusion. Apparently, one of the Peytons used her middle name also when she started, but dropped it (IMDb says she’s registered with SAG as Peyton Elizabeth List, but dropped the middle name before the younger one began her career).
It also doesnt help that, per Google’s AI, “the two actresses actually appeared as a regular and an extra in the exact same scene on the soap opera As the World Turns. Years later, they stayed at the same hotel and kept receiving each other’s voicemails and call sheets.”
There’s a whole trope about actresses who look like Margot Robbie, including Jaime Pressley, Samara Weaving, and Emma Mackey.