<iframe
 src="in_iframe.html"
 width="100%"
 height="200"
 scrolling="auto"
 frameborder="0"
>

IE6 displays (unnecessary) horizontal scrollbar every time vertical scrollbar appears.

Workaround

1. iframe: scrolling="no"
2. page in iframe: put its content into container of the same size as the iframe
3. and set its style="overflow:auto;" (overflow-x: hidden; overflow-y: auto;)