The most common mistake I see is writers who are in a hurry to get to the "good stuff" of their story. We've been taught over and over again to start our novels with action - and for most of us, that's when the fun starts! So if we're to start our novels with action, then it makes sense that a lot of us start our books with the Catalyst. It's not your fault if you do this - it's the fault of our education.
But IT IS WRONG.
Yes, start your novel with action as in, start it with something happening.
That does not mean to start it with your Catalyst.
What it does mean, is to start with ACTIVITY.
A reader doesn't want a passive main character, they want to see the character doing things. They want to see their life, their friends, their skills or lack of skills. Think of it as acceptable voyeurism. Be (and allow your readers to be) a voyeur. Peek into your hero's life - into all aspects of their life - and show us what it's like.
Act I is your THESIS WORLD
A thesis is a statement of truth or fact. It is the way things are. The status quo.
This Act is designed to show your reader what the hero's life is before everything changes and the Catalyst drops in like a bomb. An easy way to do this is to show your hero at home, at work and at play.
When your reader understands what the hero's life is like, they'll be just as surprised by the Catalyst as your hero and, most importantly, they'll be just as engaged and invested in the outcome.
Understanding where the hero has been, will help your reader truly appreciate who your hero will become.