1. Putting a raw egg into the radiator to seal leaks. Unlikely to seal the leak, much more likely to block the very fine water passages in a modern radiator and lead to overheating. Also most modern cars have a separate expansion tank and no cap on the radiator - putting an egg in the expansion tank will just make a mess. Either use a proper sealant like Bars Leaks, or replace the radiator (but see below for a bodge which works).
2. Using exhaust repair paste to hold a rotten exhaust together. This stuff only works on small cracks and pinholes, where the surrounding metal is sound. If the exhaust has rotted from the inside out, the 'repair' will last about ten minutes. MoT testers do not usually accept exhausts held together with repair paste or bandages.
3. Using thicker oil to quieten a rattly engine. All this will do is put more strain on the oil pump drive, which may then break and trash the engine totally. Also it will make the engine harder to turn over when cold, which is likely to finish off your battery and / or starter motor. If your engine rattles, try to find out why - it may be something simple, like a worn water pump or valve clearances needing adjustment.
4. Tightening cylinder head bolts to 'cure' a weeping head gasket. If the cylinder head wasn't warped already, it will be by the time you have done a gorilla impression on the head bolts. Once a head gasket has failed, replacement is the only cure.
5. Using Easy-Start fluid on a diesel which is hard to start from cold. Sure it will start the engine, but just listen to the clatter as it fires up. That is the sound of piston tops, valves and head gasket taking a hammering, and sooner or later something will let go. If the engine won't start easily from cold but is otherwise fine, the heater plugs probably aren't working - much less expensive to fix than a blown up engine.
6. Swapping tax discs from one car to another. In case you hadn't noticed, the police have things called computers. It takes a policeman about 0.001 seconds to find out whether a car is legally taxed or not. You might as well cut a circle from a picture postcard and stick it on the windscreen, for all the good it will do you. Oh yes, and 'Tax in Post' stopped being a legitimate excuse some time in the early seventies.
7. Filling rust holes with body filler. If you are planning to keep the car for more than a week, this is a total waste of effort. Under the filler, the rust will continue to develop, and within a few days your freshly applied paint will be covered in brown bubbles. Body filler is intended for dents and scratches - not holes.
8. The other thing about body filler is that it is only plastic, and not very strong. So if you are using it to disguise rust holes anywhere near steering or suspension mounts, or anywhere else which gives the body its structural strength, you are mad. If you are doing it on a car that you are about to sell, you are evil and should be locked up.
9. Cleaning up brake linings which have been soaked in oil or brake fluid, then reusing them. Don't. The oil will have soaked into the lining material and it will already be starting to break up and crumble. Brakes are far too important for people to take dodgy short-cuts with them.
10. Washing your car with washing-up liquid instead of car shampoo. Washing up liquid gets its thickness from the addition of large amounts of salt. So what you are really doing is washing your car in salt water. Does this sound like a good idea?
....and a couple which do....
11. If you have a hole in your radiator from a flying stone or similar, clean up the area with emery paper, then smother it in five minute epoxy. Nine times out of ten, this will seal the leak.
12. Car failed the MoT on emissions? Try the following: remove the air filter (but put it back after the test), increase the idle speed slightly (petrol engines), reduce the maximum speed setting or try using an anti-smoke fuel additive (diesel engines). The best thing to do is take the car for a really long run before the MoT and get the engine nice and hot.