- Not delegating. New leads often feel that if they're not writing code (or doing whatever it means to be an individual contributor), they're slacking off. The definition of "work" has changed. It now means "helping your team succeed". If you try to be an individual contributor, you are doing your old job instead of your new one, and you'll get overwhelmed and sink.
- Not asking questions. New leads often think that being a lead means being an expert, so they're embarrassed to ask or consult someone else about their team's domain, for fear of looking inadequate. Your team and your peers are the experts— it is your job to ensure that expertise prevails, whether it's yours or someone else's. You'll look much more dumb if you're too insecure to admit what you don't know, or fail to ask basic questions that allow you to get the job done.
- Hiding problems. When things aren't going well (and often they don't), don't hide this from the leadership above you. They are there to help you and guide you, and can save you from crashing and burning. You will look much worse if a problem comes to light months after it could have been communicated and dealt with.
- Not communicating. Share what's going on with your team. Share what's going on in the big picture. Don't keep people in the dark, either above you or below you.
Some leads (moreso if they're prone to impostor syndrome) think they have to wait for permission to speak, or to be offered something that they need— don't do this. If you have something important to say/ask, it's your responsibility as a leader to say it and get what you need.
- Forgetting the big picture. As an individual contributor, you often only have to think about near-term or small-scope problems. It is an easy mistake to make as a new lead to spend priority on those things instead of keeping in mind what's necessary for the entire team to succeed at its high level goal.
Good luck!