Most CPUs guarantee that aligned loads and stores up to the register size, i.e. now usually up to 64-bit, are atomic.

The compilers also take care to align most variables.

So while your scenario is not impossible, it would take some effort to force "a" to be not aligned, e.g. by being a member in a structure with inefficient layout.

Normally in a multithreaded program all shared variables should be aligned, which would guarantee atomic loads and stores.