If you ask for too little salary and don’t correct the error you will end up leaving a lot of money on the table over the course of your career.
It is not impossible to fix the error however it can be difficult based on the personality of the person you are negotiating with and their employer salary negotiation persona.
If you realize that you asked for too little salary, be honest and let the employer know you would like to revise what you asked for based on new market research data.
To fix the error of asking for too little salary you could say something like…
“I’m really excited about this opportunity and would love to get started on [Insert Date]. I previously asked for $X however based on new market research I’d like to revise that ask to $Y. This number better reflects what companies at a similar stage are paying for someone with my skills, experience, education, and track record of success.”