It's very easy to make these mistakes and you'd be suprised at how many people do actually make them:
1) Not conducting retrospectives
Not having regular meetings so everyone can understand any mistakes they performed in the previous Sprint to learn from it is a mistake that sometimes happens because you might feel like they're a waste of time but they can help boost up the teams effiency and communication.
2) Acting as a Mediator to the product owner and developer
Acting as a mediator can delay the process of development as the Scrum Master may not always be available and could have lots of issues that others could deal with in the team.
3) Being the assistant of the team
Some Scrum Masters might believe that they are only there to assist the team but that would create unreliable team members who cannot do things on their own and they'll rely on the Scrum Master for any help which leads to them not being confident to do it on their own.
Thank you so much for reading and I hope you have a great day!