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Potential file corruption

I am worried that files may be getting corrupted on my 2015 MacBook Pro (Retina). I worked on a Word 2011 (version 14.7.4) document on my Mac running OS X Yosemite 10.10.5 for several hours and then hit Command-S to save. Suddenly parts of my document were in Chinese characters.

It looked like this, with English characters intermixed with Chinese (this displays the same in post preview on Safari on my Mac as it does in Word, so hopefully others can see it):

seen t 漀爀 椀渀 愀 洀甀猀攀甀洀 挀愀琀愀氀漀最甀攀⸀ 圀栀攀渀 䤀 昀椀爀猀琀 挀愀洀攀 愀挀爀漀猀猀 琀栀攀 椀洀愀最攀 椀 琀 he caption informed

I poked around in Word and found no setting or command that seemed related to this. I confirmed in System Preferences that I do not and I think never had a Chinese keyboard enabled. All typing in the document, other Word documents (I had two others open that look fine) and in other applications was unaffected. After poking around (I'm not sure if this was true immediately or happened a little later), I noticed many menu commands were grayed out in Word and Word also stopped letting me enter text (although I could place the insertion cursor) the either the messed up document or the other documents I had open.

I closed everything and opened Word again. Now everything seems normal, and I can enter text without an issue. But that one document still has Chinese characters where my writing used to be. I copied and pasted the text into TextEdit and also opened it in LibreOffice and in both cases it looked the same.

I have four questions:

1. Has anyone ever encountered this behavior before? At first I thought I had accidentally pushed some keyboard command while trying to hit Command-S instead, and turned on some Chinese language feature. But this similar problem:

https://superuser.com/questions/907701/content-suddenly-converted-to-strange-let ters-after-save

makes me think that it is in fact a file corruption issue that occurred when I hit save. Which is frustrating because how can I save my work regularly in order to avoid data loss when the act of saving it corrupts the file!?

2. Once a file has become corrupted like this, is there any way to undo it and get my writing back? Word's Undo command had no effect when I tried right away. Is this a character encoding issue or something else that I can somehow reverse and recover the roman letters?

3. I understand my software (Word 2011 and Yosemite) is somewhat out of date. I've been working on an "if it's not broke, don't fix it" principle, particularly because I am a grad student and would like to avoid any interruptions in my work unless it's necessary. However, if this is a bug in Word or Yosemite, then it may make sense for me to upgrade. If this is a known bug, is it fixed in later versions?

4. Any other thoughts about what I can do to prevent this happening in the future? This case wasn't so bad, because it was only a few hours' work, but it was discouraging and it would be much worse for it to happen to a longer document. I am now worried about doing future work on this computer because I don't want to lose it. Any guesses about whether this kind of corruption would be a one-off thing, or should I start worrying about drive failure or other serious problems? I just ran Disk Utility and it says the volume appears to be OK.

While I was messing around after it happened, I did get an error that "There is insufficient memory or disk space. Close extra windows and try again. (AutoRecovery save of [my file name]...)" I have 45 GB of free disk space so I don't think this is a disk space problem. In addition to Word, I was running several other programs (PowerPoint, Rhino, SketchUp, Spotify). But I have 16 GB of RAM and I feel like my MacBook Pro should be able to handle running those programs without corrupting my files. I need to run these programs for my work, so I really do not want to have to be afraid that I will try to save a document I have been working on for months and potentially corrupt it. I use Time Machine regularly but I really do not want to lose the work that I would do between backups if there is anything that can be done to reduce the risk of corruption like this happening in the future.

Thank you!