Hmmmm...mistakes....so many potential mistakes. Over-sharpening images, using too much noise reduction, being afraid to move the ISO to 400 to 1600 in bad light; stubbornly sticking to ISO 100 or 200 when that means blurry shots due to too slow a shutter speed; failure to use flash or a reflector in many situations where the light is weak or poor; not making backups of precious digital files; shooting in JPG-only mode on important shoots; failing to understand the camera's operational modes; not reading the manual fully; a number of other bad habits can be found.
Good habits? Knowing the camera's various modes; making backups; shooting in RAW capture mode; using flash as needed; learning how to develop RAW Files using Lightroom; learning how to approach shooting situations with the knowledge learned from the millions of others who've gone before you; realizing that shooter education and practice is the key to improving one's photography more so than new gear or new lenses; learning how to SEE lighting conditions, and learning how to USE LIGHT in different ways; understanding the fundamentals of photography as taught by the late great John Hedgecoe in his 32 how-to books on classical photography.
Photographic technique and theory has not changed much in 50 years; the **cameras** are different, yes, but photography as taught in the Hedgecoe books is a field of endeavor. Newer books and YouTube are long on digitial imaging concepts, but lean on photography techniques and theory and practices and ways of approaching LIGHT.
Learn about photography from the books of the previous 50 years. Photography is the key to making good pictures. Digital imaging is the tools and the software, but not the theory of the field. Digitial imaging is to photography as the microwave oven and frozen box meals are to fine cuisine and French cooking. Most beginners get wayyyyyyyy too obsessed over digitial imaging buzzwords like "workflow", and "Photoshop", and so on, when what they really ought to focus their efforts on is classical photography concepts, like perspective; depth of field; lens selection; composition; wide-angle lens use; telephoto lens use; basic optical fundamentals like extension tubes and focusing properly, and routine field methods like tripod use, and flash bouncing, and how to get people to relax in front of the camera, how to direct people, and so on.