Oddly enough, restrictive bowden tubes. The "tractor'd" filament becomes wide enough to add resistance to filament feed... sufficient to finally jam in the tube. It would happened consistently with some prints where a lot of pull-backs (de-string operations) would take place along with many dot depositions on supports, for instance. Where the filament would run back and forth with the driver gear over the same piece of filament half a dozen times and it flattens to a wider state than some narrow regions within the bowden tube. Add heat from friction and plastic expanding and you got a nice jam going that will have you chasing your tail for a while. Teflon is not nearly as slick as people want you to think it is. Our plastics exhibit significant stiction to Teflon.