This happened to me and a guitar amp, but I suppose it could apply to cars, too. A while back I bought a new (well, technically used) guitar amp. I was online trying to learn more about it, and the experts said the same thing: To really kick it up a notch, install this super-awesome upgraded part.
Retail was like $25 for the part. That's it for this super upgrade? Sign me up. Of course I hadn't installed it, and tonight I was checking out the amp a bit closer. (Hey, been busy lately.)
So, what did I find upon closer examination of the amp? Yep, that super-awesome upgrade had already been performed.
Anything like that ever happen to anyone else?
The build code in the glove box of my 88 Silverado indicates an open differential with 3.73 gears.
Had it in the shop for rear brakes and axle seals, was considering adding a limited slip while the rear end was opened up.
Turned out to already have one. Not a gov-lock either, but a proper clutch type LSD.
Was all hot to buy a big bore kit for my motorcycle.
After reading up on how it would run after the modifications had been done, I'm fairly certain it has already been done. This thing was somebody's hotrod at one point and has everything else already done to it.
Not going to tear into a perfectly good, running engine to confirm.
After I blew up the differential the first time in my money pit 1999 Cobra, I decided that deeper gears would really let that DOHC 4.6L sing. Since I had to replace everything, including the diff housing and halfshafts, I thought it would be a good opportunity to put 4.30s in.
Well, after the shop had done all the work and I got to see the pile of destroyed parts, what do I see stamped on the nuked ring gear?
Yep, 4.30. 
If there ever was a car that nearly led me to financial ruin multiple times, that was it.
Wait a second, Doc. What do you mean I've already had a vasectomy?
gunner
Reader
10/3/17 7:05 a.m.
I was going to buy a turner motorsports chip for the M5. fortunately before I ever ordered one I decided to reflow the solder on the ecu. When I opened up the ecu it already had one. Thank you previous owner!
My '98 Buick Regal had the Death-Cool coolant already replaced with more normal stuff.
I'm definitely the obsessive type that always verifies what I have before I buy stuff.
What were you replacing? Preamp tubes?
Crackers said:
I'm definitely the obsessive type that always verifies what I have before I buy stuff.
What were you replacing? Preamp tubes?
Reverb tank. So I guess that I now have a backup.
D2W
HalfDork
10/3/17 10:20 a.m.
AngryCorvair said:
Wait a second, Doc. What do you mean I've already had a vasectomy?
Sometimes it's best to make sure.
Trans_Maro said:
Was all hot to buy a big bore kit for my motorcycle.
After reading up on how it would run after the modifications had been done, I'm fairly certain it has already been done. This thing was somebody's hotrod at one point and has everything else already done to it.
Not going to tear into a perfectly good, running engine to confirm.
What kinda monster are you? If it ain't broke, try harder
Believe me, it doesn't get babied. 
The 2013 BMW 328 I bought earlier this year already had Bilsteins on it when I got it. Didn't realize it till a week later. While I wasn't planning to do any suspension mods to the car, it was a nice surprise. Wish they would have done springs too.
I bought a Lockrite differential locker for the Samurai, to install when I swapped in the 5.30:1 centers I had bought. Disassembled the rear diff, it already had a spool in it. Figured I'd stick the Lockrite in the front, disassembled the front diff, it already had a Lockrite.
I think this is similar and happens all the time to all of us: buy a part, wait a year before you decide to get around
to installing part, can't find the part, order another part, install, find old part. arrgghh!
