This known issue only occurs under the following circumstance. Manually type a URL with parentheses, then click "Shrink It". The resulting short URL will redirect to the wrong source page. This is because by manually typing (as opposed to copy and pasting) a URL and shrinking through ow.ly causes the parentheses to disappear. We are currently investigating the cause of the problem, and when we have discovered it, we will submit a fix.
In the meantime, if you need to shrink a URL via manual type, we suggest replacing ( with %28 and ) with %29.
We will update this article again once a fix is in place.