I had one project where I estimated and gave a "project price" based on that. Mid way through (right after I wired up the payment provider) the client found out they were not approved to use that provider. So now I had to delete a weeks worth of code and start over.

After the change I sent a revised bill, and the client ghosted me.

Specs change, but people don't understand. If you have a wood framed house and mid way you decided it should be metal framing it changes the price.