I have been editing lecture videos in Kdenlive on Linux, and encountering a frustrating problem: a significant fraction of the time, after I get to the end of the video, I find that somewhere along the way, the first few seconds of my timeline got deleted. I'm sure this is a result of accidentally hitting some shortcut at some point, but I have not been able to figure out where in the last hour I did something wrong. Can anyone help me identify what might be causing me to accidentally cut & delete something at the beginning of the timeline while working much later in the timeline?

My general workflow mainly involves going through a couple of long clips from start to finish and cutting things out. This involves a lot of switching between X and S modes, frequent use of the delete key, and a lot of ctrl-z/ctrl-shift-z to undo/redo. I also end up using M-mode, group/ungroup, and change-speed at some point in just about every video. Any ideas how I might be accidentally modifying the start of the timeline in this process?

Thanks in advance!

Without watching what you did I cannot help with the accidental cutting and deleting.
For your workflow I have some suggestions:
* Do your first "scan' of the footage in the clip monitor and use In and Out points to bring segments into the timeline
* Use markers and/or guides while scanning your footage to aid in editing later
* Move cut segments you are fine with into a different video track that you can lock later while working on the other tracks
Workflows are very subjective - what works for me may not work for you, and vice versa

In case anyone else encounters this in the future: the problem was the Z shortcut for "LIFT from IN to OUT point", which I was pressing accidentally when trying to press either X or ctrl-Z. I've now disabled that shortcut, which seems to have solved my problem.