Here are some mistakes that should be avoided by the designer no matter what:
Some designers think that just because a certain company or business is small and doesn't have much popularity, they copy off of their work. This is plagiarism and is a crime as you are stealing something that belongs to someone else. A professional designer knows that he should not copy or steal someone else's designs. Designers put a lot of time and effort into designing and their clients put a lot of money to hire these designer so one should never steal as they have worked so hard to get their own design. Some designers do it and don't tell the client and then it becomes the clients' problem when they are caught. The client will have to pay a lot of money for this problem and the designer will have just gotten away with it.
One of the most important factors of a successful design is that the design should be kept simple so that it is easy to scale. If the designer designs an overly-complex design, it will be very hard to scale it and will cause a lot of problems then. Complex designs may seem like a good idea but they are easily memorable as well. It is more easier to remember a simple design than a rather complex one. Some agencies and companies do have complex designs, but it suits them so it looks good otherwise it would just look terrible. A complex design could also make customers confused as to what kind of company it is.
Picking a color or color scheme is extremely critical as what color you pick will have an effect on the way the audience views the company. Color can also be used to convey a message to the audience and if picked correctly, it could send a powerful message to the audience so a designer should be careful when choosing the colors. The designer should also be careful that the logo design doesn't depend on the colors only. The logo should look great in any color.
A designer should be careful with the typography of a logo because just like color, one important aspect of a logo is the typography. If you use cliche fonts like Times New Roman or Arial, then the logo could be ruined and look like its designed by an amateur. The designer should also be attentive of the spacing and should not use more than 2 fonts in a logo or it will be ruined.
Some designers have the tendency to be selfish. They want their portfolio to look good so that more people hire them and do not care about the clients' wishes and needs. If they have made some sort of similar design in the past, they will not make it again, despite the fact that the client wants that kind of design and that design will look good for the company. They don't want people to think that they have one style only and will do anything to maintain their portfolio.
Some designers include Co., L.L.C. and other such stuff in the logo design. This unnecessary stuff should be removed to make the logo simpler and easier to remember. If the client wants to add these things, then it should be done but only if it looks good otherwise, the designer should let the client know that it will look bad.
Just because some software is really popular on the market, the designer shouldn't use it if it is a bad software for logo designing. The designer should use what he is comfortable with and what he thinks will work best for the logo design.