Now you see it, now you don't - can a nozzle unplug itself?

Bambu Lab P1 Series Troubleshooting
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I had to stop a print last night after about an hour into a 11 hour print. I was getting ready for bed and casually looked at the job before turning in. I was surprised to see that a lower portion was suddenly looking bad in an area that was nearly completed (see photo). I got up this morning and tried printing a test piece about the same size with the same filament, same settings, same process settings and it turned out great. What the heck?

I’m still relatively new to this game, but from looking at many troubleshooting discussions, and a bunch of photos, it looks like maybe a problem with under extrusion, perhaps a clogged nozzle? There were very smooth lines right up to the problem (see photo). So, can a nozzle unplug itself, or did something else happen last night? BTW, I’m currently running another print of a FreeCad model of mine, and again, everything appears to be smooth (at least halfway through the print job).

I’ve never had a nozzle clog before or seen this sympton on my printer. I even hate to post this, because there are so many opinions for causes out there that can overwhelm a person trying to get a little help. I’m thinking of doing a cold nozzle pull just for GP.

P1S printer bought in Jan. that only has 454 hours on it
Textured PEI plate
Door closed, top slightly open
Using dried Petg HF filament kept in AMS with lots of fresh desiccant or dry box with same, below 20% RH
Filament profile: 230C nozzle, 75C bed (initial layer), 18mm/s max volume, Aux fan turned off, no cooling 1rst 7 layers
Process profile: Modified .16mm process - 60mm/s top surface, slowed acceleration to 5000, ironing, etc. - this is a well used profile with me. See photos of problem, and next day a similar print. Thanks for looking.

the problem
the problem1920×1080 216 KB

Next day print
Next day print1920×1080 73 KB

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Brother, welcome to FDM. where reality and sanity occasionally take a break to dance naked in the moonlight.

I print sets of stuff often, anywhere from 3 to 20 sets sometimes individual plates, sometimes plate after plate of the same group of pieces that together make an assembly.

I haven had a piece jettison off the center of the plate, landing a good 2 feet outside of the chassis… literally watched it happen live., about halfway through a print, the other 8 identical items went on to print perfectly.

I have had a print I do often just randomly fail, and not just fail, a catastrophic parts flying off and clown hair everywhere.

Less often with Bambu printers, but in my 12th year of playing with these I will say… It sometimes makes my wee scientist brain cry in the corner.

I am super pedantic about note taking etc, first as an officer, then as a systems consultant, and eventually while getting a degree in chemistry it served me well, now I just print out pics of the wierder bits and stick em on the wall.

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