What’s the biggest mismatch you’ve seen, heard, or done?

It happens. Whether from ignorance, expediency, transition, or plans interrupted isn’t important. Power, class, gain, size, impedance, cost, and so many other ways an audio system’s parts can differ from expectations. For years I had a CD player that cost more than my speakers and receiver combined because my intended upgrade path was put on extended pause. Right now I can’t decide on a preamp so I’m using one that cost only 1/10 of what my amp did. I see pictures of enormous speakers shoehorned into the corners of tiny rooms or someone using a flea watt amp on a power hungry speaker. I’m sure someone has used a cable that cost more than the components it connected. I’ve read about numerous impedance or gain matching issues. The objective is to entertain and educate rather than just mock (that’s going to happen a little no matter what, but try to be nice). So what’s your story?

I paid $264 for my vintage speakers. They were an eBay purchase and I picked them up from the seller while traveling near his home for a family ski trip. They are paired with an integrated amp that is a current model and retails for $3k. I also have the matching CD play/DAC that also retails for $3K. Add in the $1k I have in my turntable, and that's $7K in electronics, playing through $264 speakers. A mismatch made in heaven.....
My first wife.  That was a mismatch from hell.  Thank  goodness my 
second wife worked out so well.  No upgrades required!
I once had what looked like a mismatch on paper. A pair of magnepan 1.6s with a 100
watt Music Reference RM9. This system was in a large room and I listened at fairly loud levels. It shouldn’t have worked, but man it sounded glorious.

Tried using an Almarro A318A integrated with Harmonic Technology Pro-9 9-gauge speaker wire. Sound was totally muffled -like having two blankets over the speakers. Switched to Nordost Blue Heaven and sounded like, well, heaven.
The Almarro was only 18 wpc and the HT works beautifully with high-powered solid state amps. But if there is ever any doubt that speaker wire makes a difference...
The worst mismatch i have ever heard was with a pair of pair of spendor s100 speakers on a mark levinson 334 amplifier i almost did not buy them because i thought they sounded that bad but it turned out to be just how bad the amplifier was and it was downright aweful.
Tube Audio Design Hibachi amps (no feedback) driving kef ls50s. Yuck. Completely flat and lifeless.  Big time impedance mismatch. 
Counterpoint SA7 Tube preamp w/Perreaux PMF 1150B SS amp. That was my education in impedance synergy!
Seen: 1200hp Allison in a 914.
Heard: Ruger Blackhawk hand load, beside my Glock 17.
Done: Mother in law, in my 911.
My worst mismatch was Topping DX7 Pro or Topping E30 (as Preamp) and NAD M22V2, they made the speakers I had by then (Klipsh Heresy 4, Revel Salon 2, Revel M16, Tannoy Kensington GR, Monitor Audio Bronze 2) sound bright and fatigue. The problem is the NAD M22V2 which is on the bright side of neutral and very transparent, so pairing it DAC Direct (DAC with digital volume control to be used as preamp) is not a good choice. It is much better to pair the NAD M22V2 with Moon 390 or Anthem STR Preamp, which are built as preamp.
CJ preamp and SS amp.
Do you have any idea why they paired so poorly?  I’ve seen lots of other tube preamps paired well with SS amps, so that can’t be it.  Do you think it’s specific to CJ?  Their tube pre’s are usually very good.
I had thought 'sonic mismatch' but see monetary ones as well.

Sonically, an old ARC LS-3 with a Threshold T200 power amp and Martin Logan SL3s. I forget what the T200 replaced but it was a lesson about auditioning first, especially at that high price (even back then). It was listenable for twenty minutes at best. Sold the T200 very quickly because it was a great amp, just not with ML speakers. An oops moment/decision.

Monetarily, a guess my lovely burl Raidho D2s paired up with less than $6k retail worth of source/power electronics/DSP/cabling. Oh, plus three good subs ($2200) for 25-35Hz reinforcement. To me, the sound is from heaven, the best I've had after forty ears despite owning any super $$ electronics. (Yes, if I had the money it could sound better still, but diminishing returns sure set in.)

The biggest one was an entire trend/movement:

B&W 800 and Matrix series speakers with Audio Research electronics.
Years ago I went into Shelly's Audio in Van Nuys and they were using a pair of Maggie 3.something, powered by a pair of VTL Tiny Tot amps that produced 25 watts per channel. Gotta say, within their power range, the system sounded amazing. Then they switched the amps out for a pair of VTL Ichiban amps. Still amazing ... and a lot louder too. 

I'd love to try a pair of the Tiny Tots on my Legacy Signature IIIs ... 94db efficiency.