Sometimes you'll notice the side effects, like if you have a small OLED or LCD screen and start getting garbage characters in your strings.

Even if you do have an MMU, there's no guarantee that you'll get a segmentation fault from a memory bug. You can still just get the weird side effects, if you fail to access the incorrect memory.

Undefined behaviour means exactly that. You have no idea what you could get.

I was once working on an embedded system which did not have segmented/paged memory and had to debug an issue where memory corruption preceded an uncommanded reboot. The root cause was a for-loop gone amok, intending to loop through a linked list for ever member of an array of somewhat-large structs. The terminating condition was faulty, so this loop would write a garbage byte or two, ever few hundred bytes in memory, right off the end of the 32 bit memory boundary, wrapping around to the start of memory.

But because the loop only overwrote a few bytes and then overflew large swaths of memory, the loop would continue passing through the entire address space over and over. But since the struct size wasn't power-of-two aligned, eventually the garbage bytes would write over the crucial reset vector, which would finally reboot the system and end the misery.

Because the system wouldn't be fatally wounded immediately, the memory corruption was observable on the system until it went down, limited only by the CPU's memory bandwidth. That made it truly bizarre to diagnose, as the corruption wasn't in any one feature and changed every time.

Fun times lol