Insert following two examples to some wikipage and compare the outputs:
Hello :: Hi ::: Would you take a look on this code? :::<pre> code </pre>
Hello :: Hi ::: Would you take a look on this code? :::<source> code </source>
As you can see, the source tag is correctly indented, but the pre tag isn't. This behavior looks like a bug. If this behavior is intensional, then it should be reconsidered.
Currently there are some templates (like this one on cswiki) used as a hack to indent pre tag somehow, but it is not ideal.