Programming embedded systems from scratch. All hardware, memory, timers, peripherals, etc, must be set up correctly at startup, and if you set even one single bit incorrect in any of the sometimes hundreds of 32- or 64-bit configuration registers, you are screwed. There is often no terminal that prints error messages to help you, but if you are lucky you have an (often very expensive) hardware in-circuit debugger to step through the start up code.