Hi, I have found a few others who have had this issue with the TwentyThirteen theme but as yet have no resolution.
If the sidebar content is longer than the main content then it overlaps into the footer. See http://www.themaynard.net/weddings/civil-partnerships as an example.
I am guessing/hoping it can be resolved by modifying the CSS but despite numerous attempts I cannot resolve it.
Thanks in advance of any help.