1. If you add a "ret", you just jump to an invalid address.

2. If you add nothing, the CPU will continue to execute the bytes that follow.

In both cases it is quite certain you end with a segmentation fault.(Or in case 2., an illegal instruction)

You keep running whatever ‘instructions’ appear in the data after your last program instruction, so you could most likely have a segmentation fault, or worse run some random data as instructions and so crash, or worse still run some random data that happens to also be real instructions that does something harmful.

Segmentation violation.

I've seen code with a hlt instruction after main and the exit system call. Not sure what their intentions are, it should be unreachable.