We are not perfect and to make perfect systems can take forever. So in between the compromises we make, we accumulate tech debt in some way or the other, maybe by not upgrading critical software (because it works!).
EDIT: Wow, this looks like such an interview question, I realized!
Mark
formerly known as M
I think tech debt is more like an accumulation of delaying small and large improvements hundreds of times. It comes from being short-sighted for years.
So I can't really point to a few decisions that are the worst tech debt. The parts are just bugs, bad architecture, lack of unit tests... But individually they are not examples of tech debt, the same way that a tree is not an example of a forest.