Two things I find difficult

In web development:

• synchronizing state between frontend and backend
• distributing state inside a frontend application

In embedded software:

• getting the LED to blink at the right speed
• I2C

In software architecture:

• communicating with people who don't think in systems
• getting team buy-in while keeping a coherent vision

In data science:

• cleaning and indexing CSV files
• recognizing and explaining the 800 ways data can lie

When learning:

• finding the right balance between practice and theory
• making up exercises when none exist

(titile image: wolfgangfoto - CC)