A new intern can lose a good chance not only by doing poor work. Everyday behavior also matters. Internship mistakes often begin with a phone, a careless attitude, or too much waiting for instructions. In an office, people notice these things, even when no big project is going on. Five common habits can make the experience less useful and leave a poor impression.
The first problem is giving attention to something else while at work. A supervisor may pause a conversation to answer a call. This is not spare time for playing a game or writing messages. The intern still needs to be ready when the conversation starts again. Keeping the phone away helps avoid an awkward moment. Even when nobody mentions it, repeated actions can change how a supervisor sees an intern over time.
Music also can separate a person from what happens nearby. Some offices allow earphones, especially during quiet tasks. But using them all day may mean important office talk is missed. Low volume or one earbud makes it easier to hear a colleague calling or news about available work.

Being sleepy is not the same as sleeping at work. An intern may arrive tired and find the morning difficult. Resting the head on a desk, however, is visible to others. The original problem then becomes a matter of behavior. The person should manage tiredness without using the workplace as a place for a nap.
Lack of initiative causes another difficulty. When assigned work ends, there may be no next task ready. An intern who simply waits can look uninterested. Asking a nearby colleague about unfinished tasks gives a better picture of willingness to work. Even ordinary help is part of office life.

Casual dress does not mean every outfit suits the workplace. Clothes that are too revealing or impractical can attract attention for the wrong reason. Looking at what colleagues normally wear gives a simple guide, though it is not a written rule.
These mistakes have little to do with academic knowledge. They concern attention, readiness, and respect for shared space. An internship can include quiet hours and easy assignments, but the way an intern spends those hours still leaves an impression.