Website migrations, redesign and SEO

I wanted to see what type of SEO issues people encountered during their redesign or website migration projects. I am working on a software to automate the QA process and mitigate the risks. I would like feedback from others like if you noticed any drops in traffic, or there were bugs / issues like canonicals went wrong, URLs changed, content got messed up, etc.?

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What are the common issues you encountered during a website migration/redesign?