Hi Allen_K,
We are still fighting this issue where HTML keeps getting reformatted after hitting Post. The most common thing I have seen is where blank lines are being removed and it is extremely hard to get them to stay via the Rich Text Editor.
Please see this link. I just spent about 20 minutes trying to get the Rich Text Editor to quit deleting my blank lines but to NO avail. Most posts do not do this but sometimes it does and when it happens you cannot get Rich Text Editor to do what it is supposed to do no matter how hard you try.
When you hit Post you can literally see the blank lines being removed before your eyes before it returns you to the thread. The ONLY way I have found to get around this is by using the HTML Editor directly.
This problem is pretty annoying. 
I have also seen that it appears that the use of Spoiler tags makes this problem worse, though this is not the only time it happens.
One thing I can say however is that in the few times I have used spoiler tags this problem ALWAYS happens, at least so far.
Perhaps the thread I linked to above will provide a clue to the core problem?
Thanks much
Allen