Having been an engineer for the past 15 years, I have committed some and seen other people make some big mistakes inadvertently. Some of them are listed below.
1. Being a specialist
A specialist is a employee who knows everything about his/her field. The whole company comes to a specialist for advise regrading his area of expertise. For instance, if you're a IT expert working in a automobile factory where you handle all computing issues anyone in the company might have, you will become an indispensable asset to the company.
However, this also can be a disadvantage. While you may be a specialist for your company, your personal growth will stagnate as you'll have no new skills. Thus, as an engineer you should always delve deeper and either develop a present skill or develop a new skill.
2. Increasing efficiency of learning
Lets assume you want to learn JAVA in the next six months. You can either learn JAVA by reading a good book or joining a course at a training institute. Which method is more efficient and which offers more value in the long term? Isn't discussing JAVA concepts with a senior developer at your company a better idea, considering you will learn more in one hour about the expected level of knowledge you should have to code JAVA for a company. Always optimize your own learning process.
3. Focusing on job title
Many engineers leave their companies after being unhappy with their growth prospects and career development. Leaving a job is not a sin, but doing so without proper planning and though is downright stupid. Evaluate your position at a company at regular intervals. Look for reasons to stay at a company rather than to leave, as this will force you to look inwards rather than blaming others