Generally I find that many companies have a small amount of difficult problems. The trick is to become the person who gets to solve them when they come up.

At one of the places I worked, I got to implement a request batching algorithm using dynamic programming, at another place I got to implement a nurse rostering algorithm (this is not as easy as it sounds, and is an active research area), and at a third place I implemented the recursive tree algorithm that keeps a lookup table of user permissions up-to-date in a situation where roles inherit permissions from other roles.