Download, never "open in"
Update: the described behavior depends on the browser. I was using SeaMonkey. Chrome browsers don't seem to place the document in /tmp. Firefox makes the opened document "read only".
Here's something really stupid that once every few years catches me unawares:
I download a document from Google Drive in .odt format.
The dialogue box asks whether to "open the document" in LibreOffice, or to save it.
I click to open in LibreOffice.
I work on the document. Looks fine; it has a name and everything, and I carefully save it before closing.
I turn off the computer.
I open the computer the next day.
The document has vanished.
It went into /tmp/mozilla_ , a folder that is completely erased every time the browser is closed or the computer is shut down (I'm not sure, but the result is the same).
Of course, the solution is not to (not ever!) "open the document in LibreOffice" but to save it, then open it. But every so often I forget that. And then, like now, it can result in the loss of several hours work. There are no warnings against this behavior, either in the browser or in LibreOffice. It's something you just learn the hard way.