The cause: Say you edit with TextWrangler. You save your most recent changes, and Transmit tells you it has uploaded the file, but the changes don't appear. Critical point: You then close your current TextWrangler editing window for the file, and reopen the file directly from your server. However, you find that only a small portion of the file has been transferred back to the server after your last update, leaving you with only part of the file now open in TextWrangler and no way to Undo, because you closed the window/file previously.
So what do you do?
Well, caches can come to the rescue. All files that Transmit transfers for external editing are cached (for how long, I don't know), under ~/Library » Caches » TemporaryItems » Transmit » .abcd » filename.ext. Lucky for me, I found a cached version of my file inside one of those hidden directories. I used Terminal and pico to check each directory for the file, and then moved (and duplicated) the golden file to my Desktop once found.
The file I found was the original file immediately before the last save that was uploaded to the server. I do not know how persistent these Caches are, but I suspect they are time-sensitive as some of the hidden directories inside the directory are actually empty. Good luck, and I hope no one ever has to use this hint.

