What happens on a stack overflow, and how does it differ between a desktop and a no-MMU MCU?

A stack overflow is when the stack grows past its allotted region, from unbounded/too-deep recursion or oversized local arrays (or VLAs with a large runtime size). On a desktop, the OS places a guard page just past the stack limit; touching it triggers a fault the OS turns into a clean segfault that terminates the process at the point of overflow, so it's relatively easy to diagnose. On a microcontroller with no MMU, there's no guard page: the stack just keeps growing and silently overwrites whatever memory is adjacent, .bss, the heap, or in an RTOS another task's stack. There's no fault at the moment of corruption; instead you get bizarre, delayed failures far from the root cause. That's why embedded developers size stacks deliberately, avoid deep recursion and large locals, and often fill the stack with a sentinel pattern at boot to measure the high-water mark and detect near-overflows.