You can receive this error message for a number of reasons, ranging
from posting
to the wrong site to various manifestations of a change in the
attribute set or category.
Detailed Description
If you see this error, here's a Knowledge
Base article which gives general advice on how to troubleshoot this
issue. This article discusses a specific case where an attribute
changes from a "Site-Wide" AttributeSet id to a non "Site-Wide"
AttributeSet id for some categories.
You can also see this for more basic category changes. For
example, you may see this if you're using attributes in a sub-category,
and that sub-category was rolled up into a parent category (eliminating
the sub-category).
Category changes happen on a routine (approximately monthly) basis.
This Knowledge
Base article discusses how to keep up with category changes.