I've been working on an instructional video for the last two days. About ten minutes long. After doing some cut edits around the 5 minute mark, I zoomed out of the timeline and saw that the first 4 minutes worth of clips where gone! What the heck?

I tried undo-ing back to see what might've happened but kdenlive crashed.

EDIT: The deleted clips were all in the timeline 'zone' -- it seems likely that I accidentally pressed 'z' while I was zoomed into another part of the timeline, and so didn't notice kdenlive delete hours of work. That's a bloody dangerous shortcut, if you ask me, especially given the propensity to crash when undoing. I changed the shortcut to something that isn't a single key. I guess the rest of this post is moot, but i'll leave it in case anyone wants to address.

Does anyone have any suggestions on how I might try and restore those clips?

I'm using 17.12.1 and MLT 6.6.0 on Linux. I have also noticed that the Project Monitor will show clips that do not exist in the timeline, and that if I save and re-open, the timeline reflects what I was seeing in the Project Monitor (in other words, not how I had left it before I saved and closed the software.) I've also noticed that the clip thumbnails in the timeline aren't always accurate and sometimes show another clip (though the correct clip plays if you scrub over it.)

All of this leads me to ask... what is people's confidence level in this software? Do I have to treat it like an ancient version of Premiere on System 7 and do a 'Save As...' every time I make a change so I can be sure it won't corrupt my work? It is by far the nicest video editor to use on Linux but I'm pretty hesitant to touch it at this point.

I have encountered exactly the same problems several times now.
I really like Kdenlive and find its workflow to be intuitive and fast, moreso than using PowerDirector which I am falling back on.

In my current project, I found several instances where a particular clip magically appeared in my timeline near the end of the project. It appeared again after deleting it.

Several times I have lost clips from my timeline, but for what reason, or how, I do not know. They are simply gone and I don't know which processes caused them to disappear. Now I'm trying to import those clips from a previous saved version using the Library feature, but the Expand Clips feature does not work and I can't get the clips into my current project.

I'm hoping that version 18 will be far more stable and resolve the problem of disappearing clips. For now, my next few projects are going to be in another program.
I can't afford to spend many hours making a video only to have it scrambled up just as I'm about to complete it. But look forward to the day when I can use Kdenlive for all of my projects. My thanks to the team.

Settings > Configure Shortcuts > Search for "Lift timeline zone" > Press on "Z" > Choose "Custom NONE" > Press OK.

Done. Now that shortcut is removed