There is a rule that expalin that a variable can not be upadated twice in an expression. if such things happen the result depends upon the implementation of that specific compiler .

Answer 1

No, there's no such rule.

You can write x=a++ + ++b + --b + --a;
variables 'a' and 'b' were updated more than once, and there is no mistake in this expression.

Answer 2

As, Aseem said there is no mistake in his statement.

But if the statement only consists of a single variable that is incremented or decremented multiple times then the variation starts to occur.

Eg:

x = ++x + x++ - --x + x--

In this statement x will have different values for different compilers, I.e. it is compiler implementation dependent