Sending the same email to every company on a list. Managers receive enough email to recognize a template, and an internship cold email that could have been sent to any company in the industry tells them you didn't care enough to research this one.
Hunter's State of Email Outreach found that 69% of decision makers are bothered when they suspect an email was AI-generated. Hiring managers are exactly that audience. But the problem isn't using AI. It's using it lazily. Instead of generating a generic pitch and blasting it to fifty companies, use AI to research each one. Pull what the team shipped recently, understand how your skills connect to their work, and write a pitch that makes that connection obvious. What used to take a full day of research now takes minutes, and the output is an email that reads like you spent real time on it.
Underselling your actual skills in favor of general enthusiasm is a close second. "I'm a hard-working, motivated student eager to contribute" could describe every applicant. "I've been working on [project] and I'm looking for a team where I can take that further in a real product context" gives the reader something to evaluate. Lead with what you can do, not with how willing you are to learn.