Andrew Novikov - Threads, callbacks, and execution context in Ruby 

When you provide a block to a function in Ruby, do you know when and where that block will be executed? What is safe to do inside a block, and what is dangerous? What is a block, after all? Blocks in Ruby are everywhere, and we’re so used to them that we may even be ignorant of all their power and complexity. Let’s take a look at various code examples and understand what dragons are hidden in Ruby dungeons.
Ruby programmers usually don't need to think much about blocks… until they have to. As a contributor to Ruby gems that have callback-based API for developers, I've found that internal implementation details of these gems affects how these callbacks are executed and resulting behavior can be quite surprising sometimes, and I think that good Ruby developer should know inner workings of blocks to better understand execution flow of complex Ruby programs.