When a forum or a topic has pages, the pagination causes a bottom scrollbar in responsive between 700px and 500px.
I think its a Chrome only issue.

The solution could be moving this in responsive.css:

.action-bar > .pagination {

		float: none;

		clear: both;

		padding-bottom: 1px;

		text-align: center;

	}

from @media (max-width: 500px) to @media (max-width: 700px)

There may be better solutions, but the above solves the issue