We can easily imagine a language that is exactly like C in every regard, but free of some gratuitous behaviors. All standard-conforming C programs work in this dialect, and a good many others also work and are portable.

For instance, we could have a dialect of C in which this is required to print "0123":

   int i = 0;
   printf("%d%d%d%d\n", i++, i++, i++, i++);

A behavior is gratuitously undefined if it is left that way for no good reason, such that programming language constructs can be undefined simply for having the wrong form. That is to say, the input values are well-defined (i has a good initial value, which we can increment four times), and the individual operations are defined also (i++ is fine by itself). But for no possible value of i is the above printf call correct.

An example of undefined behavior which is not gratuitous is overflow on integer addition. Th expression i + j, where i and j are int, is not ipso facto undefined because of its form. Only for certain combinations of values of its operands is it undefined. We cannot simply banish that without banishing addition, and various ways of making overflow defined have drawbacks, like being expensive (e.g. target machine has no native support for the particular behavior, so extra instructions have to be generated) or super-expensive, with complicated representation and memory management (switching to bignums).