Some documents require not only notarizing, but also a couple of additional, impartial witness signatures as well. This is especially common with power of attorney (POA) documents.
When you notarize any document - particularly a POA - check for the presence of witness signature lines. In some cases, instructions included with the documents explicitly state whether witness signatures are required. If they are required, be sure the witnesses are not otherwise named - and have no vested interest - in the documents.
Rule of thumb: In the presence of witness signature lines, don't assume that notarization alone is enough. If you're not sure, always err on the side of safety and include witness signatures to make sure your client is adequately protected.