For example, I built a distributed video transcoding system a few years back, and for the use case at the time it worked great. But let's say people want a new video format? That's a large rewrite. Want to add more servers into the worker pool? That's a large rewrite (the code was written to assume a 1:1 ratio of certain servers, which was dumb).
Basically anything that you might want to change was made harder by the code that I wrote to get the job done initially. Over the years you start to recognize that, and write your code a little smarter and more change tolerant (hopefully introducing less technical debt).
