Technical Debt: we all have it. Yet, this phenomenon remains poorly understood by product managers. Unlike financial debt, the costs are often hidden and difficult to measure. But the most dangerous aspect is that "Technical Debt items are contagious, causing other parts of the system to be contaminated with the same problem, which may lead to nonlinear growth of interest." 
Here's a case study of one such event; unmanaged tech debt caused interest costs to spiral catastrophically out of control.