For me that would be Young Rascals

3 Dog Night around 2000. Only one dog showed up with a backup band. NOT the same!

major bummer-- it's like when Mike Love is the Beach Boys frontman

hope they played eli's coming. great song
 

They played like a good cover band but NOT the original. That was around 2002. I saw them in the late 60's and early 70's. They were great!

Spinal Tap Mk II - I caught them at the Themeland Amusement Park in Stockton, California. 
 
 https://www.youtube.com/watch?v=JtvGR8UX1L0
 
 

Queensryche
B52s
Billy Idol (was great in 1990, awful in 2005)
Guns N Roses (early 90s, the performance was fine but a fan got onstage and security held him down while Axl kicked the s**t out of him. Lost all respect for GNR in that moment)

America, opening for the Doobie Brothers, whose FOH guy either intentionally sabotaged the PA for them or just set everything at 0 and went for a beer. The band was clearly not having any fun, going through motions just to get through the set so they could split for the casino. (The Doobies, OTOH, despite also having played their material 20,000 times, still manage to put on a slick, energetic show.)

I expected this thread to be created to go along with the others.
 
Many may tread on me for this one: John McLaughlin, Al Di Meola, Paco De Lucia
 
Damn. What a sleeper event. As a guitarist I thought this show might be great. The three came out to an empty stage with three chairs. And then performed for about 45-minutes as an acoustic set. Ok, I was fine at that point. Then they said they were then taking a short break. I figured during that break the 'band' would be setup and we could get into some 'electrified' sounds and percussion. Well after 20-minutes or so, the stage curtain went up and what do I behold? Three chairs on an empty stage. Talk about your heart dropping to your stomach. And the three then proceeded to another session of Latin based themes, super flash guitar leads, etc, for another 45-minutes or so. I fell asleep at some point.
 
I have to say though, the rest of the crowd seemed to love it. Maybe today I would appreciate this concert. Just back then (25-years?), it just wasn't my cup-o'-tea and not what I expected. 

bayoubill
3 Dog Night around 2000. Only one dog showed up with a backup band. NOT the same!

I saw them once sometime in the past 15-years also. The billing was Three Dog Night (or maybe it was Chuck Negron) and Blood, Sweat, and Tears. I figured David Clayton Thomas would have to be in the BST part. From what I recall reading once, Chuck Negron got the BST band rights. So he had them as a backup band on the same card. But, no David Clayton Thomas. It wasn't a bad show. Just a bit odd that a major band (as least during my upbringing) such as BST would be a backup band, and would not have the strongest performer in that band being a part of it. So it was a disappointment, but I took the show for what it was and was mildly entertained hearing the old 3-Dog Night and BST tunes at least.