I understand now - in the EAQ it said to use spoiler tags NOT hidden tags and then the spoiler tag changed to a hidden box... So I got sort of confused.
Anyway, I made two videos showing how block placement acts differently with forge enabled and disabled. If this is caused because the server and client disagree, then forge is causing them to disagree somehow, because the base client does not have this problem. The "desync" is not present at all locations, but it is always present at the same coordinates within a world. However, creating a new world with a new seed will change the locations at which the bug occurs. (or so it seems)
I just updated to 1206 to make these videos. Thanks for helping me clarify the issue.
[bug] With Forge
[bug] Without Forge
The videos are unlisted. I'll remove them at your request.