I started some longer (8 hours) prints with ColorFabb XT-CF20 filament and I got some layer shifting now and then. I'm using v6 with stainless steel 0.4 mm nozzle. By analyzing the parts, I observed that there are come bulges attached to the parts, nearby the layer that started the shift. Moreover, the nozzle looked very dirty, having material all around. Initially I suspected material leaked around the thread for some reasons, but after dismantling the hotend and analyzing it, there was no evidence of that. I decided to be patient and observe the printing process as close as possible and I was surprised to find the following phenomenon. Material accumulates around the tip of the nozzle and slowly rise up around the nozzle. Seems that after a while (I do not intend to recreate the full experiment, just extrapolate the observations) it creates a bulge that fall on the print due to gravity. If the print is large enough so that the bulge will cool down until the nozzle returns to that area, it will obstruct the normal print path and the layers will shift. It's a peculiar behavior that I didn't meet with other filaments. I say an older topic related to the Taulman Bridge that reported a similar issue, but it was not reported as being solved or at least explained. It looks like that surface tension is responsible for it, perhaps "supported" by the nozzle profile and certain conditions (temperature?!). (see the accumulation on the right side of the nozzle tip in the picture). Any ideas?