General
3D printing is a mechanical process. It can deal with real small stuff, .08mm layer height with .2mm lines. Very minor things can have a large impact on a print.
For example. The filament is 1.75mm. Not exactly, it does vary and a lot of websites will say something like 1.75mm + or – .05mm. This means that if the filament is in spec, it probably isn’t along the entire length of the spool, it could have 1.8mm² in a mm at the low side and 2.482609959375mm² in a mm on the high side. That’s over 30% more plastic in the fat section than the thin section. How well is a flat wall with a short section of the fat filament going to look?
A little breeze will affect how a section of the model cools, which affects stress in the plastic, which may pull a bit off the build plate, that has a bit of oil there. If it is on the edge any one of these thing could push it over. You have one print that is fine and the next one, with no changes on your part, curls. If the print curls it may get knocked. If it gets knocked you could end up with a bunch of spaghetti. That’s why you have two prints where you changed nothing and yet one has a bunch of spaghetti.