I can think of a few examples that often pose challenges for less experienced Devs, some of whom don't overcome these challenges, whether due to lack of motivation, capacity, or any other reason:

- Handle DateTime in different timezones consistently, according to the expectations of each customer and their geography.

- Proper use of caching, basic search algorithms and data structures.

- Thread synchronization / race condition.

- Anything that requires recursion.

- Database and query tunning.