You don’t need to think of anything unique or new to say with this question. It asks about “common” mistakes, so just talk about the classics. Here’s an example:
“One common mistake is to neglect or attempt to work against business needs. Sometimes UX designers have very different priorities than stakeholders, but it’s important to realize that we need to work within the system rather than against it.
A lot of the changes and improvements we propose cost time and money, so we need to convince other departments that they are worth the investment. However, if we push for things that are too radical, we can end up harming our own efforts.”
