When are you supposed to learn from your mistakes in coding / design? Is it something you take with you to the next project or do you learn in the middle of your current one, sacrificing consistency for cleaner, more well-informed code?

For example, my application can be distinctly demarcated down two lines of business -- say one side is for sales and the other is for marketing. Both are somewhat tied together, but as far as the team structure, use cases, developers, etc. the app consists of the Sales code and the Marketing Code. Now, say the Sales code went in first and while good-intentioned, made some bad mistakes. Should the Marketing Code follow suit and make the same mistakes for the sake of consistency or should Marketing architects and designers instead learn from the mistakes that Sales made and developer a cleaner codebase, even though Sales and Marketing are in the exact same system?

Basically, do you learn from your mistakes while in a project or do you continue to pile crap on top of crap?