ExpressionEngine 2.5.2 Build 20120606
The issue:
In _only_ the current production environment, two of the eight channels will not allow an entry to be modified or a new entry to be added. The behavior is that the new/edit page simply reloads again and no changes are actually submitted to the database. There are NO errors displayed on the screen or in any of the server logs (not even JS errors in any console.) We were able to produce errors related to the Matrix add-on one time, but upgrading that solved the issue that was producing those errors, but it did not solve the core problem.
Strangely, these two channels are similar in setup as two others - they use the same fieldtypes and hold similar content (but are separate/unique fields for each channel, not actually shared between them.)