Bugreport:
When adding an annotation (without appearance) and afterwards signing it, adobe reader/acrobat reports that the document has been changed afterwards when validating the signature. This is however not the case. The PDF Viewer basically thinks "ah there is an annotation missing thus i am adding one. Ah and surprise, surprise the document was changed now (by me and in memory) but still I report it..." However the actual state of the document is, that there was no change afterwards.
This bug has been described on several mailinglists (itext, stackoverflow etc) in the past, too.
See the attached example.