Not understanding what the manager wants
Sometimes, even when the interns think they've done everything right, the feedback they receive is unexpectedly bad. This happens when there is a miscommunication between the interns and the managers in terms of expectation management. To avoid that, always take notes of every instruction and follow up with the manager after hitting small milestones to ensure you're going in the right track.
Not paying attention
Different people have different preferences when it comes to how they like getting the work done. When working with different people, it is important that interns always observe and note down the preferences of the managers. Some like short emails, while others prefer elaborate ones. Some prefer calls over SMS. There are a lot of random things, that may seem harmless, but can really annoy employers and it's always a good idea to avoid doing those.
Not dressing the part
Some interns think that they don't have to dress like the full time employees just because they're interns. This is a completely wrong and dangerous perception to have. Interns should always dress professionally. First impressions are really important and as such, interns must always dress according to the dress code.
Taking the internship for granted
A lot of interns spend the entire duration of their internships slacking off, sitting on their desks, avoiding contact with everyone. Performance matters, even when it comes to interns. Thus, interns should thoroughly think about the short term and long term consequences of their performances and act accordingly.