You may have noticed that occasionally when creating your corridor surface that the contours from the corridor surface sometimes do not "match up" as expected. Contours will never “perfectly” tie in because contours are not the TIN, they are an approximate representation of the TIN. We are modeling surfaces, so it performs an exact match from edge of TIN to edge of TIN.
As you may have noticed in your corridor some places can seem worse than others. This is due to the frequency set in the corridor parameters. A very wide frequency will cause the surface creation to interpolate between those intervals, and if the terrain varies quite a bit between those frequencies, the interpolation can produce less than desireable results. Although the results appear to be incorrect, it is just Civil 3D doing the best with the information provided. The trick to resolving this issue is to shorten the frequencies in these areas and you will see your contours begin to tie in to existing ground much better.
Moral of the story: tighter frequency=tighter contours
The fodder of this post was obtained from a conversation with the one and only Jason Hickey. Thanks Jason!