db.collection.find().limit(-1) and db.collection.find().limit(1) is equivalent in mongo 2.4.7,but our mongo being used is 2.2.4.We found that sometimes limit(-1) is different with limit(1) when querying.
in pymongo,the function _getitem_ send a limit of -1 when getting one document:

if isinstance(index, (int, long)):
if index < 0:
raise IndexError("Cursor instances do not support negative"
"indices")
clone = self.clone()
clone.skip(index + self.__skip)
clone.limit(-1) # use a hard limit
for doc in clone:
return doc
raise IndexError("no such item for Cursor instance")
raise TypeError("index %r cannot be applied to Cursor "
"instances" % index)
In large skip,mongo return null,which is expected to return one document. After changing limit(-1) to limit(1),we got correct result. I consider limit(-1) is a bug,or does it exist something i did not realized?