You probably heard that Global Interpreter Lock (GIL) in Ruby "allows only one thread to run at a time", while technically it is correct it does not mean what you probably think it means. After this talk you will know why your code can be not thread safe even if GIL is enabled and how even a subtle change in your code can make GIL fail to protect you from race conditions. This talk is junior-friendly, but even experienced Ruby devs most probably will learn something new and important about GIL in Ruby.