Found the problem:
The #c2 element has 1507px height, a 30px bottom padding, and a 10px bottom border.
The .widget element has 1507px height, a 10px top padding, a 10px bottom padding, and a 10px bottom border.
After you add everything up, the #c2 element is 10px taller than the .widget element, even though they both have a height of 1507px.
It's your call on how you want to fix it, but the simplest solution is probably to just change the #c2's bottom padding to 20px.