Lost a chunk of aluminum because I got impatient mid-cycle

Same thing bit me on my ST-20 a couple years back. I was running a batch of drill cycles on some aluminum brackets and got bored waiting, so I started typing G-code tweaks for the evening job right there in the editor while the spindle was turning. Partway through the batch the counter skipped ahead and it skipped two holes. No crash, just two missing holes and a confused me holding a caliper. I did basially what you did, ran it back with edits and without. Hands off, perfect every time. Editing mid-cycle, it would glitch maybe one run in three. Never the same spot twice either, wich made me think it was something the control was doing on its own while I typed.

My theory was that the editor redraw and whatever the background was doing to the macro variables just stepped on each other, but I never dug deeper than that. I asked our Haas guy about it and he just laughed and said don't do that, treat a running program like it's radioactive.

So now I keep a printed copy of the next job's changes on a clipboard by the machine and mark it up with a pencil while it runs, then type everything in once the cycle ends. Feels old fashioned but it costs me nothing and the control stays happy. That machine has run hundreds of macros since without a single repeat of the problem.

Funny, I edit during cycles on my VF-2 all the time and never had a loop counter misbehave. What did bite me once was saving the file itself while the program was running. I had a facing macro with the counter stored in a variable, wrote my changes, hit save, and the control aparently choked on reading and writing the same file at once. Next pass the counter reset to somthing dumb and the tool dove in like the stock was still there. Scrapped a vise jaw that day, which hurt more than your fixture plate did. After that I got in the habit of saving edits under a new program number instead of over the one that's running. Haven't had a repeat since, and I still sit there typing away while chips fly. Might be worth trying that next time you want to punch in the next job mid-cycle. If the weirdness follows the saving and not the typing, at least you know what to blame. Could also just be your control is an older software rev than mine and it handles the editor differently, no way to know from here.

I wonder if it depends on which file you're editing. On my VM-2 I got in the habit of opening a copy of the next job under a diffrent program number and typing into that while the macro runs, and in three years of doing it I've never had a counter skip. The one time I got lazy and edited the running file itself, same thing happened, extra pass and a gouged vise jaw. Could be coincidence but I stopped testing that theory after the vise. Try the copy trick and see if it still misbehaves.

Count me in the same club. My fix was even lazier than a clipboard, I just keep the next program on a USB stick in my pocket and transfer it over after the cycle ends. Used to stand there editing on the control during long facing runs on my old knee mill and yeah, weird stuff would happen. Since the USB habit started, not one repeat in over a year.

The thing I noticed too was that it wasn't just editing. Even flipping through pages on the screen, like looking at offsets or positions, seemed to raise the odds of a glitch. Hands in my pockets, machine runs like a rock.