I have been a prepper since long before the term was in use, over 40 years and during that time I have seen many prepping mistakes made by others or myself. Here are a few example that may save you time and money.
1 Start out by gaining knowledge, I have seen people run out and spend a lot of money based on the recommendation of a good salesman and later regret it.  Put in the bookwork first, find out what you really need and then spend the money. Get more than one opinion, many of the so-called experts make mistakes and put out bad information.
2 Take stock of what you already have.  Often you have items in your home that can be used for pepping. I have seen people rush out and spend money they don’t have for sleeping bags and forget the trunk full of grandma’s blankets they had in the garage. Use what you have, you can upgrade later.
3 Don’t confuse having a gun collection for prepping. I know people who think they are preppers and have spent tens of thousands of dollars on firearms and ammunition, but have almost no food.  You need balance in your preps.
4 Don’t start out by stocking large amounts of food that you do not normally eat. I have known people who bought tons of wheat and then found out their system would not tolerate it. Eat what you store and store what you eat.
5 Learn to cook the foods that you are storing. If you are storing wheat, learn to make bread, now, not later. This also helps you to rotate your foods.
6 Buy the best gear you can afford, avoid many of the prepackaged bug out bags that contain cheap gear and are overpriced. Some of the best buys I have made came from garage sales and thrift stores. Often you will find older American made camping gear and tools that are superior to the Chinese junk sold in many stores.
7 Failure to train. If you don’t practice now, chances are you will fail in an emergency situation. There is a learning curve to everything. Learn all the skill you can, such as medical care, gardening and food preservation.
8 Avoid having a normality bias – it refers to a mental state, people enter when facing a disaster.  It causes people to underestimate both the possibility of a disaster occurring and its possible effects.
During the September 11 attacks, many people in the World Trade Center returned to their offices during the evacuation to turn off their computers and ultimately died when the towers collapsed.  More than 70% of survivors checked with others before deciding to evacuate. Almost any action is better than none.
9 Keep an inventory and know what you have. This can help you prioritize your spending and save you money in the long run.
10 Don’t forget about water. I have known people who spend money on food and ignored water purification. Lack of good water can kill you.
11 Keep your mouth shut. The less you tell others about your preps the better.
12 Training, I know I mentioned this before, but it needs to be mentioned twice. Practice what you learn, as you train and develop skills your confidence will increase. Even if you lose all your preps, you will still have the training which will give you an advantage over most people.
Avoiding these prepping mistakes can save you time and money.
Howard
You said “5 Learn to cook the foods that you are storing. If you are storing wheat, learn to make bread, now, not later. …”
The importance of bread making is grossly overrated. A very fine tasting porridge made from unground wheat kernels and other grains can be made by simply simmering the grains in water for about 30 minutes. Add some dried fruits and you have a gourmet delight. And all of this without yeast or baking powder, the physical labor of grinding the grain, and with much less consumption of valuable fuel.
Hangtown Frank
The forest gump adage ‘stupid is as stupid does’ is does..I have been involved in prepping since the 1970s and made mistakes that prove that the hardest lessons are the ones we learn from. An example was acquiring food for preps but not being aware how heat destroys and degrades food items. I have given advice to individuals and groups over the years about food preservation and climate effects on them.