Real dangers of 2+ threads writing/reading a variable

What are the real dangers of simultaneous read/write to a single variable?

If I use one thread to write a variable and another to read the variable in a while loop and there is no danger if the variable is read while being written and an old value is used what else is a danger here?

Can a simultaneous read/write cause a thread crash or what happens on the low level when an exact simultaneous read/write occurs?