This issue may be caused by your having more than one author with the same surname, or multiple cases of the same author being parsed as different authors by the program (for example, Smith, J, Smith, J., and Smith, John are all read as different authors by Mendeley at the current time). This will lead to the program attempting to disambiguate between the authors by adding initials or the first name. This problem can be avoided by ensuring all authors are added in the same format.
If this does not help, please send your database (http://www.mendeley.com/faq/#locate-database) and the document in which you encounter these errors to support[at]mendeley[dot]com, letting us know which citations are wrong. Thanks in advance.