When Tuxedo Moon opened and Crime headlined at Mabuhay Gardens in 1978. That was a travesty. Crime didn't deserve to be in the same city as Tuxedo Moon.

Didn't Sha Na Na play before Jimi Hendrix at Woodstock?  To me that is pretty strange.

My 8th grade girlfriend gave me tickets to see Sha-Na-Na in Chicago in 1973, and Robin Trower was the middle act before their headlining.

King Crimson played before (and in the daylight??) at the Horde festival w/ Blues Traveler and Lenny Kravitz following up (Blues Traveler was pretty awesome).
 
I think only about 40 people were there to see Crimson.
 
Stephen

Jerry Lee Lewis,
Ron and The Daytonas,
-and-
Gene Autry, The Singing Cowboy (with his horse).
 
Mid- 1960's, Ozark Empire Fair, Springfield, Missouri.
 
Jon

I've seen some pretty strange pairings in my day, Poco opening for Yes, and I did see The Who as the middle act for Herman's Hermits, The Blues Magoos were the opening act. Talk about freaking out a bunch of pre-pubescent girls!!! Vanilla Fudge opening for Cream but one of the weirdist was The Beach Boys opening for The Grateful Dead. That one was quiet a culture clash.

Brownsville Station, Mott the Hoople, and Kiss?  Man, this would have been my dream all-star concert from my junior high days!  I grew up in Iowa, so we could see Kiss a couple of times a year, Brownsville Station when they had an album out.  Never got to see Mott the Hoople, although I did see Ian Hunter after I moved to California (sadly, not the same).  I am going to go spin Mott  the Hoople Live now!
 
Worst pairing?  Around 1979, when I was at college, the student government somehow managed to sponsor a show where UFO opened for The Atlanta Rhythm Section.  Two great tastes that really don't go together.  Somebody must have fancied themselves as a junior Bill Graham.  After many years of hearing almost nothing but dull thud at Veteran's Auditorium in Des Moines, I was fairly shocked to hear some amazingly pristine PA at Maples Pavillion at Stanford for ARS.  If you remember back that far, you'll remember that the bassist was a very big guy that played a Rickenbacker.  The vocals and guitars at that show sounded very studio, but I remember that the bass was really clearly defined too, and that the big guy played a lot more than I expected.

The Dictators, Burton Cummings and Alice Cooper, in that order, for Cooper's Welcome To My Nightmare show in Terre Haute, Indiana..
 
Uriah Heep (with Wetton) opening for KISS..
 
John

One of my partners in my office went to that Kool/VHshow here in Toronto - a diehard VH fan. Described it as bizarre .

In 1974 I saw The Elvin Bishop band warm up for Frank Zappa and The Mothers.

This is a little off subject, but still a strange pairing.....I saw the Experience Hendrix Tour last night, & Bootsy Collins did a pseudo funk version of Purple Haze! It was cool seeing Bootsy for the 1st time & his famed star bass, but I was thinking that Jimi was probably rolling over in his grave with all the disco pops being played on his anthem! Don't get me wrong, I love funk & although I'm not a slapper, I still appreciate the technique & those who can do it. I just didn't feel like it fit the iconic song at all.

Megadeth opening for Metallica in '93, Mustaine almost paid with his life!

Rockpile aka nick lowe and dave edmunds opening for van morrison at the palladium in NY.
Van played 35 minutes- threw the mic down and walked off stage.
i also saw rockpile open for blondie at wollman rink in Ny's central park (remember that place bill?) and once again they out played blondie
rockpile was a great band for 3 minute songs.

197(?): Mike Quattro Jam Band (all-instrumental duo - the eponymous prog keyboardist & a drummer) opening for Muddy Waters.
1981: The OdD (new-wave - pronounced the Odd, not the ODed) opening for Vanessa Davis Band (jump blues)
1980: Humble Pie as the 3rd act between Kansas City, Missouri (a band, not the city), Angelz (never heard a band that pluralized with a z that I liked) and headliner Mahogany Rush - you can't imagine how I wished HP was 1st, so I could have missed the rest!
 
Peter

1979ish - Summerfest, Milwaukee - Dixie Dregs (w/Andy West)opening for Marshall Tucker.
 
Somewhat similar genres - but the DD's musicianship totally whooped Tuckers. I think all I could handle was 2& 1/2 Tuckers songs and then had to leave. . .