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sanjuu, This happeneded for many after Feb 24. Were you seeing these errors bet. Feb 24 and April 11? This might provide some clue as to whether these were really triggered by Panda or it is just that google was updating the WMT backend.
this might be part of the reason why sites are getting "punished" by panda (...albeit a very small part)
google obviously believes that these pages are still part of our site, or they wouldn't suddenly be appearing like this. if google is punishing thin content, then having hundreds of 404s and hundreds of non-returnables is bound to have an effect.
sanjuu, you can see the discovery dates in GWT. If you look for the discovery dates of the earliest ones, it will provide a clue.