If you use Google Docs or Office Online, or OneNote, you're used to not having to remember to save documents: mobile apps don't ask you to say that you want to keep the work that you've just done.
So when Office 2016 added AutoSave to Word and Excel and PowerPoint recently, it should have been a good thing.
No more working for an hour then losing everything because your PC crashed before you got around the saving the document (also, no more having to work out if the cryptically-named auto-recovered document that Office has presented you with is the right one).
If you ever start a new document by opening a similar document, modifying it and then saving it with a new file name, AutoSave has some unintended consequences. Your original document gets overwritten in ways you just don't expect to happen in Word, because it never used to work like that.
A lot of people re-use documents. One day I opened an old invoice to use as the basis of a new one and noticed that the details in the invoice didn't match the file name - because AutoSave had 'helpfully' updated the invoices with all the changes I made before I chose Save As, even though I didn't want them saved in the original document.
To use AutoSave safely, you need to bear in mind that it automatically saves all changes to your document, even if you only mean them to be temporary, or if you change your mind and close without saving.
Open a document and cut out some content that you want to copy somewhere else? If you don't choose undo - or use copy instead of cut - the original document is saved without that information.
Want to use an existing document as the basis of a new one? You need to get into the habit of saving the document with the new file name before you make any changes, especially if you're stripping out a lot of the document to turn it into a template you can reuse - because all the changes you make before you choose Save As are saved in the original document, so closing without saving is no longer enough to keep your document the way it was when you opened it.
For anyone who's used to documents that save automatically, all of this may be what you expect, but for people who have been using Word for a long time, this fundamentally changes the way Word works - and you have to get used to it.
Saving before you have made any changes seems strange, because you haven't made any changes - but if you wait until you make changes, you have to make the effort to undo them.
To do that, you can press Ctrl-Z to undo if you catch yourself before you close the document. If you don't notice for a while, click the Activity icon in the top right of the windows (it looks like a little clock) or choose File, Version History, View and Restore Previous versions.
The task pane will show a list of changes to the document; click to open the version you think is before the changes you didn't want, check it and then save that file. You may not see all the versions for a document if it's been a while since Word saved changes you hadn't really realized you were making; I had to restore some documents from the copies I'd emailed or backed up.
If I edit a document and then choose Save As, Word should ask me if I want to save the changes in the original document too, or if I want to have it revert to the version I opened.
The Office team changed something very fundamental without really explaining to the users what problems it might cause, making a new feature not quite as useful as it ought to be because it doesn't match the way we've learned to work.