Oh god where do I start...
My dad owned my '92 K1500 Blazer before I got it...his idea of maintenance is change the oil every 3k miles and rotate the tires once a year. Nothing else...he sells cars when they're about 3-5 years old as they're OOOOOOOLD to him by then.

Luckily I bought it at 5 years old and around 55k miles so nothing was horribly over on it's maintenance intervals or not working.
'93 Geo Tracker 4x4 my buddy had owned (and floated down a river - frame was FULL of sand) and he had gotten it from his fiance and her family. Apparently they liked to drive around with it in the summer with the top completely off and out of the vehicle...so when it rained everything got wet. The carpet
STANK! I pitched the carpet and liner and suddenly that thing smelled like a new vehicle! There was mold growing on the passenger side floorboard!

They're still my friends but I'll never buy a vehicle from them...well, unless he wants to sell his '73 Javelin.
'99 Dodge Ram 1500...mostly just POS Chrysler engineering but when I first got it there was a sort of bog when you pushed down on the gas at 40+mph. Computer showed no issues, ran and idled great...oh wait, somehow it had a baseball sized wad of blown in house insulation jammed into the air filter box.
'96 Ranger 4x4...kid left it pretty much alone in the time he owned it from '99 to '03 when I bought it but the ONLY maintenance he did was regular oil changes and to have the front steering linkage completely replaced. Did I mention he put 110,000 miles on it and it had 140,000 miles when I got it? ORIGINAL SPARK PLUGS AND WIRES! Smallest gap was .1" and several were wider than my gapper could measure. Most of the plug wires were stuck to the boots. Wheel bearings, races, front u-joints, front shocks, front coil springs, tires so old and abused they had cracks in the tread, a whole host of sensors (went from like 8 CEL codes to 1 by the time I was done) ANNNNNNND...he loved the Silver Lake Sand Dunes so this thing had sand EVERYWHERE! I could vacuum the hell out of it every week and still come up with sand inside. You couldn't take a bolt off the thing without having sand filter down. I owned it for 3.5 years, wash my vehicles fanatically, and still couldn't get rid of all the sand.
Good truck though after I got it fixed up.
Lastly, my current '96 Tracker. Prior owner was just smart enough to be mechanically dangerous. His "mechanic" was even worse.
-Stereo "System" with like 10' of extra monster cable coiled up and taped to the bottom of the passenger side seat. Two 10" subs just rattling around in the back and an amp sitting on the tranny hump in front of the rear seat. Gee...for some reason the subs didn't work half the time. Blown front speakers because he decided little 4" speakers could thump. The wiring...amazed he and I didn't have a fire before I ripped it all out and rewired the stereo to a factoryish setup.
-Filled the transfer case with ATF instead of 90w...and then when a funny whine developed instead of CONSULTING THE OWNERS MANUAL WHICH LISTS WHICH FLUIDS TO USE they filled the transfer case about 8" higher with more ATF by putting it in through the speedo gear hole. Thought I was loosing fluid from the transmission until I realized it was full and the level in the t-case was even with the speedo gear.

Drained, refilled with el cheapo 75w90 and whalla, no more whine.
-Welded the rear u-joint caps to the driveshaft's cups. Had to get a whole new rear driveshaft when the u-joint failed this fall.
-Over torqued quite a few things like the lug nuts, speedo sensor bolt, and all kinds of bolts and connectors under the hood.
-Admitted when I bought it that when they put the lift on it they installed the rear control arms upside down and side to side and he drove it like that for a while before they fixed it. This means the axle would have been pulled forwards and the the arms would have been arcing DOWN.

-4.30 gears meant for stock 27" tires with 31's and he said it was slow and the speedo was "close to stock". Moron. Speedo was WAY off and the thing couldn't maintain 65mph with the windows down.
-These have screws in the air box that aren't meant to come out so you can't loose them. However someone had over torqued them and then replaced them with LARGE course thread screws and some LARGE bolts...that all managed to seize in place. Okay, so then he just runs the filter until it's battleship GREY. Well now what do you do to change a clogged filter in an air filter box that you managed to basically bodge shut?
Eliminate the problem entirely and just pull the air intake tube off the box! All the fresh air you could ever need!

-Heater didn't blow out the floor vents.
Turned out when he did his fantabulous wiring job for the stereo he pulled the carpet back from the hump and when he put it back he didn't tuck it under the floor vent ducts. It was covering them.
Oh there's more but I'd be typing for hours.
Oh yeah, if anyone is ever having a hard time choosing between a few used vehicles to buy just call me in to consult. I'll look over them, consider their issues and flaws along with their positives...and then pick the lemon/horribly owned one. Then you can get the good one.