Computer code that starts out small, and through extensions gains a few "bad practice" changes. Over the years, extensions mirroring the bad practice start adding up, until physical symptoms begin to appear. The system may become slow and unresponsive but it still work, for example, so it continues to grow uncontrollably.
Once this happens, there are no good options. You are forced to choose between the software development equivalents of chemotherapy or radical surgery.
Like other forms of cancer, early detection and treatment is the best option.
16000 lines of code full of elseif statements that takes far too long to load and display a single invoice is a fine example of Code Cancer.
The story is called "Immutable Invoices" on "TheDailyWTF"