The wax in the sun. It won't really hurt anything, but it is a less efficient way of doing the job. Wax won't set very well on 110 degree surfaces. Waxing in the sun is fine as long as the surface can stay cool. Once it gets too hot to touch it is not a good idea to wax it.
People use sponges way too often. Sponges are great at trapping dirt inside only to deposit it back on your paint and scratch the surface. They are not be the best wash tool in the world.
Using beach towels or bath towels to dry the car is not the best idea. I see professionals at detail shops using bath towels to dry cars and it makes me want to cry. They are not soft enough to not scratch the paint. They are great for drying bodies but not good for cars.
T-Shirts are not waxing rags. Those are also not soft enough to use on paint. A majority of damage can be avoided if you use the proper tools when you wash, dry, and wax. Forget about products and focus on your tools for a minute. Get rid of the sponges and dirty t-shirts and get some real tools.
People sometimes use Armor All like it isn't going to hurt anything. I'm not one of the "Fear armor all because it will crack your dash" clan or anything like that. I'm talking about the people who spray it all over the place and end up getting it on glass and plastic gauge covers. That stuff will haze plastic and become a real problem if you aren't careful where you spray it. It is dash protectant and not glass cleaner. Spray it on a towel first and then apply it to the dash. That avoids a lot of problems.
People think that there is a 'Best' wax or any kind of product out there. I think people need to realize that there is no such thing as best. There is only what works best for your particular situation. I don't care what letter the products starts with. It isn't the best just because everybody says it is. What works best for you may be something totally different. Get off the damn band wagon and think for yourselves. Mindless followers make baby Jesus cry.
I'm sure there is more. I was working on an article just like this for
OCDetails.com but I took it down because it isn't finished yet. When I get home I'll put it back up and let you take a look. I don't remember what all I had done on it, but it was probably basically the same stuff I just wrote.