yes Ive seen the acid engine wash which completely rapes the engine in every way possible, the acid seeps into bolt threads and corrodes them out - loose bolts, giant oil leaks.
I have also seen them use pure caustic ash and completely ruin the cast aluminium surface of the engine. I personally only used hydroflouric acid 10% solution to brighten it - Its very very very dangerous to humans but cleans the aluminium to its freshly cast white finish - I used to redo complete cylinder heads with it along with new plated bolts everywhere and people thought that it was a factory new head - lol.
anyway one time a guy asked for some of the same acid to wash the engine of his bike, I lectured him on the danger for 1 hour, showed him safety of prepping a litre of water with baking soda in case any acid splashes off. Guess what, a week later I visit the guy and his "naukar" had used it to shine up the engine on his Honda 50 with his bare hands and lost 8 out of 10 fingernails (moron) - he had the notion of "saab jee kuch nai onda - sab paagal bananay de treekay hain" - yep.. he was a moron.
anyway. The list is endless
diesel fuel spray on the engine and interior to shine it up - completely overlooking the fact that it can screw up all plastic and rubber.
mixing petrol with diesel for "solad pickup" - yeah right pick up like a turtle with polio now.
washing a fully hot engine with cold water spray - YEP, your engine is now warped or cracked.
locking the fan clutch on a mechanical fan with RTV silicone - I mean WHY??? tait thandaa rahe gaa injun - owner nods head like a moron idiot assuming he conquered quantum physics.
d'rect fan and thermashteat nikalo - same as above.
gear aur differntial oil change nahin hota - lol, hahahahaha
collecting used CVJ grease to use in NEW CV joints. I mean WHAT THE EFF MAN. - jenyan grease bohot achi hotee hai, jee magar ye grease mein 10 saal purana pakhana (potty) shaamil hai aur woh bhi saala moam (wax) ban gaya hai.
car ke neechay haail lagwao - zang nahin aayega (ask them what the hell is that thick coating from the factory, and why is it now melting off from the oil application)
radiator mein naswaar daal do leak band hojayenge - (radiator repair person will faint and the radiator would be disposed in a volcano afterwards) - and no leaks will ever patch up.
Use of brine to lock studs - I wanted to strangle the life out of the mechanic, that clown could not afford threadlocker (Rs 200/- for a small bottle that works for hundreds of screws) so he applied a very conc. salt solution to lock a throttle body mounting stud. His reply was - its the next mechanics headache now - absolutely no shame in poor workmanship.
and now lethal totkas
apply brake fluid on wound (propylene and ethylene glycol will cause the platelets in your blood to become like glass shards and do bad damage).