How is that conversion not entirely off-putting to your client if they did not change scope?
You - Hi. I misquoted. I want more money.
Client - How is that my problem?
You - I apologize for my mistake. I didn’t understand yada, yada, yada.
Client - Again, how is that my problem? I hired you because I know even less. You are the professional. Why didn’t you see yada, yada, yada coming? It’s your job.
At that point, what exactly do you say? I’m not a professional? I haven’t a clue how to do this job? If you don’t pay me … ? As I see it, the only response the client has to any excuse is a pink slip. I’ve been in both places, it is not comfortable. I would agree you have a chance of going to the well with a long-term client, but only once. If someone ever came to me a second time, even on a different project, it would not turn out well.