I think that I have explanations for both of these 
This happens when the gcode is inches. The gcode file leaves the machine in inch mode but the jog distance is still 100 so it tries to jog 100 inches. We need to come up with a better fix for that.
Whenever any motor is moving all of the motors will hold position which leads to the belts tightening up. I’m open to changing how this works but we need to come up with a clear definition of what the expected behavior is
Whew! That’s a relief to hear that these are somewhat known issues and not just my machine going rogue. So either I should create gcode files in mm (not as fun) or for now just do a unit conversion before moving the machine away. With regards to the other issue, I guess for now it’s better to just make any z-axis height changes before releasing belt tension? I can live with that.
Btw, is the fact that I’ve sometimes been seeing a mismatch between the actual position of the machine on the frame and its onscreen position at all related to the fact that the frame is in mm and the gcode in inches?
Potentially!
That’s something that I should dig into more. I do everything in MM so I don’t test inches as much as I should
Okay, I will see try working in mm and see the difference.
No! We need to fix it if there are issues with inches! 
The behavior of the machine should be the same in either unit
Yes, that’s true. That would be great actually. I personally find working in inches much more intuitive. 
Bar wrote:
This happens when the gcode is inches. The gcode file leaves the machine in inch mode but the jog distance is still 100 so it tries to jog 100 inches. We need to come up with a better fix for that.
we need something to watch the gcode or the internal state of fluidnc and be
able to tell us if it’s in inches or mm
There isn’t any standard gcode to do this, but possibly the fluidnc folks would
be open to some $ command to report it??
David Lang