If you could rebuild your personal project from scratch, what would you change?

This question checks whether you look back at your own work. "I think it's fine, I wouldn't change anything" is the weakest answer, because it suggests you learned nothing from the project.

A good answer usually has three parts:

• One specific decision you would change, with the reason. For example: "I'd write the table of examples and the tests before writing the function that reads amounts. The first time, I wrote the code first, and the message 'train ticket 32K' was read as 32 billion đồng."
• Something you would keep, to show you can tell good decisions from bad ones. For example: "I'd still reject a message that contains two amounts, because recording the wrong amount is worse than asking the user to send it again."
• A lesson that carries over to other work, such as cutting scope early, thinking about the server's time zone, or asking real users for feedback sooner.

Avoid two traps: criticising the technology you chose without being able to say why, and describing a change so big that it sounds like throwing everything away and starting over.

Bonus: prepare this answer in advance for the first project on your resume, because interviewers often ask it right after "tell me about your project".