Here is the list of top 10 mistakes that you should avoid while planning to migrate to Office 365.

Mistake #1: “Who is in charge?” – Choose your project manager wisely 
Mistake #2:  “What have we got?” – Audit your current systems.
Mistake #3: “I wasn’t told” – Involve all departments/areas of your business
Mistake #4: “Who is doing what?” – Plan your migration well
Mistake #5: “How do I log in and use Office 365?” – Training
Mistake #6: “I only needed my 2019 files to start with” – Prioritize 
Mistake #7: “I can’t find all my emails” – Prepare and test your Office 365 destination
Mistake #8: “How did he see my files?” – Maintain Security and compliance
Mistake #9: “It’s so slow on the new system” – Network and Bandwidth
Mistake #10: “I can’t get in to the office” – What are you leaving behind?

Source: Migration Monster