Got unexpained "access denied" troubles keeping people from your content? Go to admin/content/node-settings, and try the 'Rebuild Permissions' button.
If that doesn't work, see other ideas for access denied for visitors and access denied for admins.

It is odd that the settings were lost. One possibility is that they weren't set up correctly from the start - though why FeedBurner was initially working then becomes a mystery.
The other, more likely, possibility is that while trying to troubleshoot the issue of my broken subscription block, I munged the FeedBurner settings. In fact, it's quite likely:
At admin/build/feedburner, I can see my Drupalace feed all set up and ready. But say I want to check its settings to troubleshoot a problem, so I click that feed's "edit" link. There I see the fields for specifying the local feed URL and the FeedBurner feed URL – but instead of displaying the previously-entered values, the form shows the fields as empty! If I take care to click "Cancel" or just close the page, all will be well, but if I carelessly click "Save" just to exit the form, I'll have erased my settings.
It's a usability flaw in the form, and I'll bet it's where I broke things by not paying attention.
If you were using the 'Full HTML' input format, I believe it will still strip some tags that could cause XSS (form tags, JavaScript, etc). You would need to create a new input format (what I call 'Raw HTML') without any filters applied to it.
That advice is spot-on, and I'm happy to report that I was using a custom input format just like yours, which I label "Unfiltered HTML"... except – ack! – it wasn't so unfiltered after all.
Thinking I'd better check one more time before pleading "huh, I'm sure I did this right", I looked at my Unfiltered HTML filter settings just now... and "HTML corrector" is checked. It should not be!
This HTML corrector hasn't messed up most of the HTML code I paste into blocks, but I'll bet it's precisely what mangled the more complex Google search and FeedBurner codes I had trouble with. Let's check:
Disable "HTML corrector" in "Unfiltered HTML"... Create new test block using that troublesome FeedBurner code... Save block... Edit block...
And this time, the code is impeccably intact. No more mangling! Looks like I just found the should-have-been-obvious source of the problem.
So how in the world did I make this mistake? I thought for sure I'd confirmed that the input format was kosher; I must have done so for another site.
At least it's good to see that so many problems can be traced to a single cause: one big doofus user. I hereby proclaim that the "ACE" in this site's name stands for "A Comedy of Errors". : /
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Got unexpained "access denied" troubles keeping people from your content? Go to admin/content/node-settings, and try the 'Rebuild Permissions' button.
If that doesn't work, see other ideas for access denied for visitors and access denied for admins.
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