MelissaLB - 5:22 pm on Jun 27, 2011 (gmt 0)
We have been through this same problem a few months ago, still finding some issues though. We eliminated about 10,000 of these urls found in WMT with the use of 301's to the correct pages.
We still havent found our root cause but i have a very strong suspicion it is happening because of a missing forward slash in the beginning of a linking url, as someone previously suggested.
I will mention that fixing these issues did not get us better ranking but it did clear out about 1/3 of our indexed pages from google.