My preferred definition of technical debt is:
Technical debt is existing (technical) structures that stand in the way of making the next change. The interest is the cost of having to deal with those structures in your day-to-day work, and the down payment is the cost of making the structures apt for use.
Debt can be intentional in that you take shortcuts to meet an important deadline or to hit a market window. In this case you have an expected ROI for the loan you take, one that you hope will pay off more than the cost of later fixing the shortcuts taken.
Debt can also be the unintentional, and here it gets interesting. One type of debt is that which results from sloppiness, usually resulting in a messy code base. This type is not related to any kind of investment, hence there is no expected ROI, it is just like a loan for consumption of drugs; it will destroy you (your code) and your economy if it doesn’t stop.
By the definition above, debt can also be the result of external events, sometimes out of your control. Your perfect system can suddenly become your largest debt if you go from one to several customers, the company strategy changes, the market changes or laws change.
Some people, among them Uncle Bob in this article, argues that messy code is not debt. I disagree, for the reasons above. I think Uncle Bob and others confuse debt that results from loans taken to make an investment, and debt resulting from neglect, lack of experience or incompetence. The cause is totally different, but you’re still in debt.
There is no reason to make a messy system, since there simply is no return on that investment. Accept debt only to take loans that enable you to make a sound investment.