Using free services versus paid. There are many free services that you can use to create your citations and reference list items; however, some of those services are less than accurate. For example, in Son of Citation, in June 2013, if you enter the ISBN of the APA Publication Manual, Sixth Edition, it shows the author as A.P. Association rather than American Psychological Association, and indicates it is the 7th edition. Anyone writing papers conforming to APA will have this book, so, try it yourself. If you used this erroneous entry, your instructor may mark it wrong.
Most of these problems are usually obvious. If it looks wrong, it probably is wrong. Use another source, for example WorldCat.org, or enter the data into the Citation Wizard directly. There are many sources out there and they can save a lot of time; however, use them judiciously.
When you use Snatch-A-Citation, it will not check the validity of the reference beyond being well-formed, so you must be the final arbiter of any use of references captured from other sources.