Hey Bill, thanks for your question!
I’m not a hiring manager, so I don’t have the authority to call anything a “mistake”. But I will link you to a relevant Twitter thread I stumbled upon recently, where hiring managers have chimed in: https://twitter.com/jmspool/status/991669951558832128.
I think what makes a good portfolio is a good story. Process > pixels. If all that’s there is “I worked at X company, worked on X feature, and this is what the outcome looks like”, that’s not going to cut it. Even if you're strictly a visual designer - there are key decisions you need to make, and I want to know how you arrived at those decisions.
Design is non-linear and messy as heck. I want to hear about it! What assumptions did you have? What hairy questions did you encounter? What hard decisions did you have to make? What did you learn?