Alice Cooper. 
The first few times I heard Coldplay’s “Vida La Viva” (or whatever it’s called) I thought to myself “Wow! U2 finally have the jumped the shark.”
I used to get UB40 and English Beat confused a lot.
I don’t listen to Coldplay very much, but, more than once, they’ve struck me as sounding like U2 at first blush.
Early (pre-Unforgettable Fire) U2 and Simple Minds sound incredibly similar. I blame Steve Lillywhite. But it’s a great sound.
I can see The Selecter and The Specials possibly conflated with The Beat. But I find UB40 very roots reggae-oriented, while the other bands are really more ska.
Interpol and Editors. So much so that I looked like heck for that Editors song that had the line about “…three stowaways…” only to discover it was by Interpol.
Off the top of my head:
I thought the first few times I heard it that “Why I’m Here” by Oleander was a previously unreleased Nirvana track.
The singer for Meg and Dia sounds a hell of a lot like the singer for Flyleaf.
I lost $20 betting that “In the Still of the Night” was sung by Robert Plant. Turns out it was David Coverdale doing his damnedest to be Robert Plant.
Wow.
Boston is light, simple, melodic rock. Kansas is heavier, intricate, semi-progressive rock.
I don’t understand how these two bands could be confused - they sound nothing alike.
I thought the last Katy Perry singles (Thinking of You) sounded exactly like Alanis Morisette, which was surprisingly unlike Kissed A Girl.
Then I found out the connection. Sometimes, the producer is very influential, I guess.
If anything, I always thought Little River Band’s “Lonesome Loser” sounded an awful lot like Kansas’ “Carry On Wayward Son”, at least as far as the vocal harmonies.
Muse sound just like Radiohead.
I was absolutely certain- for years, in fact- that Come Up And See Me (Make Me Smile) was by Bob Dylan and not Steve Harley & Cockney Rebel (of whom I’d never heard until a radio DJ back-announced the song and I looked it up on Wikipedia).
I’m not either of the people you’re replying to, but I tend to lump most bands with geographic names together.
Back in the late-90’s in high school, I could not for the life of me tell one nu-metal band from the other. I’m sure there are all kinds of nuances that fans pick up on easily, but the whole nu-metal craze was essentially one big band for me.
There are a lot of minor groups who I just lump under the general label of “Nickelback.” The most recent examples are Shinedown and Theory of a Deadman. Past offenders are The Calling, Staind, and Puddle of Mudd.
the lead singer from Green Day calls them Cher-Bands (while doing a Cher impression, he sings out the names) i.e.
Stain’d
Nickelbaaack…
When we were growing up, my teenybopper, Rick Springfield-loving sister dismissively referred to most music she didn’t like as “beard music”…her criterion for a band being “beard music” was “they have beards.” Thus, ZZ Top and ELO were lumped in this rather broad category.
Not too long after, my sister’s beloved Rick Springfield showed up on a TV show wearing a beard. I laughed and laughed… 
Lead singer of Journey and Rod Stewart sound the same.
Before I realized the differences between their styles, I couldn’t tell them apart.
I think Steely Dan and Van Morrison are the same (sucky) band. 
“Black Betty” by Ram Jam always seemed like a Lynyrd Skynyrd song.
“Walk of Life” by Dire Straits sounds like a Bruce Springstein song.
Some of the 70’s David Bowie songs and 70’s Rod Stewart songs are pretty similar.
A lot of the old Doo Wop bands like the Platters, Penguins, and the Coasters are largely interchangable, same sort of sound and music.
I’ll admit, I don’t know all the songs or their whole catalogues. I’ve never even listened to an entire album by any of the three. But all of their “power ballad” stuff sounds more or less the same to me. I eventually remembered that “I Want to Know What Love Is” is REO Speedwagon (God, I hope that’s right). Sometimes, if I’m paying attention, I can pick out Steve Perry’s voice, but I can’t always remember if he’s with Journey.
For awhile, I couldn’t tell the difference between Moody Blues and Emerson, Lake & Palmer. Which is utterly ridiculous, I know. I blame Knights in White Satin and Lucky Man, which, for a short time, seemed like the same song to me (which is also utterly ridiculous. Stupid brain!).
I would add She Wants Revenge to these two bands.